AFTER the report of Wilder1(1952) of the finding of organisms resembling Toxoplasma by microscopic examination of chorioretinal lesions in over 50 cases of inflammatory eye disease, a collaborative study on toxoplasmosis has been conducted by the Ocular Research Unit of Walter Reed Army Medical Center and the Laboratory of Tropical Diseases, National Microbiological Institute. The purpose was to identify by other means the organism seen by Wilder and to ascertain the importance of the parasite as an etiological agent of uveitis. It became apparent in this and other studies that it was not possible merely by serological means to establish unequivocally the identity of the parasite. The high prevalence of serological evidence of past Toxoplasma infection in the general population of the areas studied prevented a positive diagnosis in the individual case. It was recognized that a combination of clinical, parasitological, and pathological evidence would be necessary to