The 55 toxoplasmin-positive patients found in a population of 698 mental defectives have been scrutinized for evidence of toxoplasmosis. Most of the patients were children. The incidence of positive cases increased with advancing age. In two cases eye lesions suggestive of toxoplasmosis were found. Radiology and lumbar puncture showed no findings suggestive of toxoplasmosis. In no cases could the mental defect be definitely attributed to toxoplasmosis. In somne cases other definite causal factors were present-for example, epiloia, phenylketonuria, mongolism. It was concluded that in mental institutions or elsewhere a positive toxoplasmin reaction is acquired like a positive Schick reaction with increasing age, and is very infrequently associated with clinical toxoplasmosis; and that toxoplasmosis is not a common cause of mental defect. Our thanks are due to Dr. I. A. B. Cathie and Dr. J. A. Dudgeon for provision of toxoplasmin and laboratory data, to Dr. 0. D. Fisher for the results of skin testing, to Miss M. Ross and Mrs. L. Mundy for psychological data, to Dr.

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