The preparation and uses of toxoplasmin, a skin-testing antigen prepd. from toxoplasma (RH strain), is described. Organisms, as may be obtained from the peritoneal exudate of intraperit. infected mice, are sedimented, resuspended in saline, inactiviated by u.-v. energy, broken-up by repeated freezing and thawing and finally preserved in phenolized saline. Depending on standardization in guinea pigs and on human beings, 1:1,000-1:10,000 dilutions of the sediment are used as skin testing antigens. Toxoplasmin evokes reactions of a delayed type of hypersensitivity in infected guinea pigs and in certain patients. In the latter, correlation with the presence of Toxoplasma-neutralizing antibodies is high. The ease of performance and the rapidity with which clear-cut results are obtained make the toxoplasmin skin test the most useful single aid in the diagnosis of past and possibly persistent toxoplasmosis, whenever isolation of the organism is not feasible. At San Francisco, Cal., the incidence of toxoplasmin sensitivity was 10% in 50 young healthy adults (20-35 yrs.); 28% in 50 older unselected patients (50-83 yrs.); 71% in 28 patients with chorioretinitis (mean age 23 yrs.); and 33% in 40 patients with anterior uveitis (mean age 35 yrs.). The use of patient''s ventricular fluid in tests for antigenic content (on infected guinea pigs and normal controls) is described as an aid in making or disproving a presumptive diagnosis of neonatal toxoplasmic encephalitis.