Acid phosphatase and glucosaminidase activity is present in the Mehlis' gland of Haematoloechus medioplexus as observed through the light microscope. At the level of the electron microscope, acid phosphatase activity is associated with multivesicular bodies found in those secretory cells of the gland that produce membranous bodies. Acid phosphatase activity is also evident in the plasma membranes of these gland cell ducts. The multivesicular bodies are believed to be derived from the Golgi areas of these cells based on results gained from the use of Golgi area markers such as osmium staining and thiamine pyrophosphatase activity. The bodies are considered lytic in function and are probably associated with regulating secretions by providing a mechanism to control overproduction of secretory products. The dense bodies produced by the second type of secretory cell associated with the gland react positively for macromolecular diglycols. On the basis of these results, it is concluded that these bodies are responsible for the mucus secreted by the glands into the ootype. Burton (1960, 1963), using the light microscope, reported that the secretions of the Mehlis' gland cells of Haematoloechus medioplexus are periodic acid-Schiff positive and diastaseresistant. In a subsequent electron microscope study, he (1967) described two types of secretory cells comprising the gland in this organism. In the micrographs of the cells that secrete a membranous body, there are evident various cytoplasmic entities that strongly resemble multivesicular bodies found in secretory cells of vertebrates (Smith and Farquhar, 1966; DeDuve and Wattiaux, 1966). This investigation attempts to ascertain the physiological role of these multivesicular bodies, as well as to furnish, by means of histochemical and cytochemical studies, some insight into the nature of the two types of secretory bodies elaborated by the Mehlis' gland cells of H. medioplexus. MATERIALS AND METHODS Adult worms were removed quickly from the lungs of freshly pithed Rana pipiens and transferred to 0.69% NaCl solution. The worms were fixed immediately in 3% distilled glutaraldehydecacodylate (0.1 WI, pH 7.2) at 4 C for 1 to 2 hr. During the fixation period, the region containing the Mehlis' gland was excised from some of the worms, sectioned on a Smith-Farquhar Tissue Sectioner (TC-2) at 50 ,, and these resulting sections (as well as whole worms) rinsed in several changes of cacodylate buffer for 24 hr at 4 C. Received for publication 20 January 1970. * This investigation was supported, in part, by research grant AI-08058 from the NIAID of the NIH. Light microscopy Cryostat sections (4 ,u, -20 C) of worms were picked up on cover slips and subjected to the following techniques. Acid phosphatase (AcPase): Sections were incubated for 10 min (pH 5.0, 37 C) in a modified Gomori medium (Goldfischer et al., 1963) containing b-glycerophosphate as the substrate. The reaction product was visualized by exposing the sections to a dilute ammonium sulfide solution. b-Glucuronidase (BGase) and N-acetyl-b-D-glucosaminidase (NAGase): Sections were incubated for 30 min (pH 5.2, 37 C) in the medium of Hayashi et al. (1968) using either Naphthol-ASBI-/8-glucuronide or Naphthol-AS-BI-N-f-D-acetylglucosamide. Hexazonium pararosanilin was used as a simultaneous coupler in both instances. Following incubation in the appropriate media, the sections were rinsed in water and mounted with glycerogel. Control sections were made, omitting the substrate from the incubation medium and, in the case of AcPase, using 0.01 A sodium fluoride.