Total mg myoglobin of mouse diaphragm muscle infected with Trichinella spiralis was less than that of diaphragms from uninfected animals by day 12 postinfection (PI) and remained significantly depressed to the end of the 24-day study. Total mg free creatine and phosphocreatine of trichinella-infected diaphragm muscle were significantly less than those of uninfected animals by day 12 PI and continued at a lower level than that of uninfected mice to the end of the experiment (day 28 PI). Total mg sarcoplasmic-granular (S-G) protein of diaphragm muscle infected with T. spiralis was significantly greater than that of uninfected control animal diaphragm muscle between days 19 and 31 PI. The total mg myofibrillar-nuclear (M-N) protein of trichinella-infected mouse diaphragm was less than that of diaphragms from uninfected animals between days 10 and 16 PI, but was about equal to that of control mouse diaphragm between days 22 and 31 PI. The wet weight of diaphragm muscle infected with trichinella was significantly less than that of diaphragm muscles from uninfected animals between days 7 and 10 PI, but was significantly higher than that of control animal muscle between days 16 and 28 PI. There was no significant difference between the dry weight of trichinella-infected and uninfected mouse diaphragm muscle between days 4 and 28 PI. These findings are evaluated on the basis of observed changes in other biochemical parameters and are discussed in terms of a general hypothesis presented in an earlier paper. Muscle fibers infected with Trichinella spiralis undergo dramatic ultrastructural (Fasske and Themann, 1961; Ribas-Mujal and RiveraPomar, 1968; Despommier and Purkerson, 1974) and biochemical changes (Stewart and Read, 1972a, b, 1973a, b). In the present study alterations in myoglobin, free creatine, phosphocreatine, sarcoplasmicgranular and myofibrillar-nuclear proteins, and wet and dry weights of trichinella-infected and uninfected diaphragm muscle were investigated. MATERIALS AND METHODS The source of trichinella, method of excystment of muscle larvae, and the procedure for infection of experimental animals were those used in previous studies (Stewart and Read, 1972a). Male 6-week-old Swiss white mice (Texas Inbred Mice Co., Houston, Texas) were used in all experiments. All experimental animals were infected with 1,000 larvae. The anthelmintic methyridine was given on day 12 postinfection (PI) to remove adult worms from the intestines of experimental animals (Stewart and Read, 1972a). In experiments I, II, and Received for publication 17 January 1974. * This work was supported by grants from the NIH, U. S. Public Health Service (2T01-AI 00106 and AI-01384). t Deceased. IV, 3 uninfected and all infected mice were given methyridine on day 12 PI. In these experiments uninfected methyridine-control animals were killed on day 16 PI. In experiment III all infected and all uninfected mice were given methyridine on day 12 PI. Extraction and determination of total myoglobin of diaphragm muscle was by the method of Reynafarje (1963). For determination of free creatine, mice were anesthetized with chlorobutanol (Eastman Kodak Co., Rochester, N. Y.), and their diaphragms rapidly removed, immersed in liquid nitrogen, and placed in dry test tubes in a liquid nitrogen bath. After freezing, diaphragms were individually removed, weighed, homogenized in 2 ml of 10% trichloracetic acid at 0 C, and submitted to the extraction procedure and assay of Ennor and Stocken (1948). The method of Ennor and Rosenberg (1952) was used to determine total creatine and phosphocreatine in diaphragm muscle. Sarcoplasmic-granular and myofibrillar-nuclear fractions were extracted from diaphragm muscle by the method of Monckton and Nihei (1966). Protein was assayed by the method of Lowry et al. ( 1951). All chemicals were reagent grade. Student's t test was employed to evaluate the significance of differences.