Since the first published record of proved human toxoplasmosis by Wolf and associates in 1939 (13), reports of this disease have appeared in the literature with increasing frequency. This has been made possible by the demonstration of the organisms in monocytic phagocytes, development of neutralization antibody and complement-fixation tests, transmission of the condition to experimental animals by infected body fluids or tissues, and better appreciation of the various clinical aspects of the disease. Up to February 1947, Miller (5) was able to collect a total of 38 cases of human toxoplasmosis from the literature. Since then 2 additional cases have been recorded (3, 11). It appears probable that infection with Toxoplasma may not be excessively rare, especially in the United States, since it has been described as a spontaneous infective agent in a variety of species of mammals and birds from many parts of the world (13). Dogs, rabbits, squirrels, rodents, and various birds, including the domestic pigeon and chicken, are known to be natural hosts for this organism. Toxoplasma is a protozoan parasite of uncertain classification, usually classed with Sporozoa. It infects many tissues, the central nervous system frequently being involved. In smears, the parasites are well outlined, crescentic or curved masses of protoplasm, 4 to 6 micra in length and 2 to 3 micra in width. In sections of fixed tissues their appearance is somewhat altered and they appear as rounded or oval types, measuring 2 to 3 × 1.5 micra. They may also occur as large rounded masses of closely packed organisms, sometimes referred to as cysts. The lesions most commonly encountered are small focal inflammatory nodules, with or without necrosis, in brain, spleen, liver, and lung. The infiltrating cells are predominantly lymphocytes and large mononuclears. It is believed (9) that the two most likely modes of infection of infants are prenatal transmission and contact with the organism during passage through the birth canal. Four distinct clinical types of toxoplasmosis are now recognized: (a) the congenital or infantile type, with the infection occurring either in utero or shortly after birth and giving signs of cerebral injury, such as convulsions, hydrocephalus, microcephaly, mental retardation, chorioretinitis, or calcification of irregularly distributed granulomata occurring within the central nervous system; (b) an acquired disease in older children characterized by acute encephalitis; (c) an acute form in adults, with pneumonia, and at times a picture resembling typhus or spotted fever; (d) a group in which high natural resistance to the organism has resulted in the production of neutralization antibodies in the serum with an absence of any clinical signs attributable to the toxoplasmic infection. The two cases presented in this report belong to the congenital or infantile group.