Autoradiography was used to study the incorporation of tritiated thymidine and adenosine into the schizonts of Eimeria tenella developing in the chorioallantoic membrane of chick embryos. 3H-thymidine was not incorporated into the developing parasites, but was incorporated by host cell nuclei. 3H-adenosine was taken up by host cells and utilized for nucleic acid synthesis. The parasite utilized this adenosine before or after its incorporation into host nucleic acids, and/or its metabolites for synthesis of DNA and RNA. It may also have utilized 3H-adenosine directly from the allantoic fluid before entry into the host cell. The only reports concerning the requirements of eimeriine Coccidia for nucleic acid precursors are those of Roberts et al. (1970) who reported no incorporation of 3H-thymidine by Eimeria callospermophili during its asexual development in vitro, and Ouellette et al. (1973) who investigated the incorporation of 3H-thymidine, 3H-cytidine, and 3Huridine by the asexual stages of E. tenella cultured in embryonic chick kidney cells. There have been no reports concerning the utilization of purine nucleosides by any species of Eimeria. This present work was undertaken to study the utilization of exogenous 3H-thymidine and 3H-adenosine by E. tenella growing in the chorioallantoic membrane (CAM) of chick embryos. MATERIALS AND METHODS Preparation of sporozoites Oocysts of Eimeria tenella were harvested from heavily infected ceca of Ranger cockerels on the 7th day after infection. After sporulation in 2.5% (w/v) aqueous potassium dichromate solution, the oocysts were filtered through a 40-/um mesh sieve, concentrated by centrifugation, and treated with 3% sodium hypochlorite solution at 4 C for 10 min (after Wagenbach et al., 1966). The resultant sterile suspension was pelleted by centrifugation, and the oocysts were floated on sterile 20% (w/v) aqueous sodium chloride solution. The harvested oocysts were washed repeatedly in sterile phosphatebuffered 0.9% (w/v) saline at pH 7.6 until the smell of hypochlorite was removed. Sterile sporozoites were obtained from these oocysts by the method of Long (1970). Infection of chick embryos White, fertilized, Apollo eggs were incubated at 39 C for 11 days before a suspension of 5,000 Received for publication 27 August 1973. sterile sporozoites of E. tenella in 0.05 ml of buffered 0.9% (w/v) saline (pH 7.0) was introduced into the allantoic cavity. Five-hundreths ml of antibiotics (2,000 IU of penicillin and 2,000 /xg of streptomycin) were inoculated at the same time. The eggs were then incubated at 40 C. Introduction of radioactive material Twenty-four hr after setting up the infected embryos either 1 mCi of SH-thymidine (specific activity 2,000 Ci/mM) or of 3H-adenosine (specific activity 5,000 Ci/mM) were injected into the allantoic cavity of the eggs. These eggs, as well as nonradioactive controls, were examined at 2 and 5 days. Preparation of material The CAM's were removed from the eggs, washed several times in warm distilled water to remove any free radioactive material and blood, and infected pieces were fixed in Carnoy's fluid for 2 hr. After dehydration and embedding in wax, sections were cut at 3 Am and autoradiographs prepared. To determine the specificity of incorporation into DNA and RNA, some sections were treated with 0.1% (w/v) DNase in veronal-HC1 buffer at pH 7 for 6 hr at 37 C. An equal volume of 0.05 M cysteine hydrochloride in 0.05 M sodium acetate solution at pH 7 had been added to the DNase solution before incubation, in order to inhibit proteolytic activity. Other sections were treated with RNase (0.3 mg per ml) in distilled water (pH 6.8) for 3 hr at 60 C, and others with 5% trichloroacetic acid at 90 C for 30 min to remove both DNA and RNA. Autoradiographs of the sections were made, using Kodak AR-10 stripping film, and were exposed for 2 weeks at 4 C. They were developed, washed free from photographic reagents, and allowed to dry for 24 hr. Autoradiographs were stained in Harris' hematoxylin. With each batch of autoradiographs two controls were included. To control against false negatives, one experimental slide from each batch of autoradiographs was exposed to light before exposure, and developed as for the other autoradiographs. To control against false positives