Serum samples from 797 healthy male and female subjects, between 10 and 50 yr old and from 3 distinctly different geographical areas in Peru, were analyzed by the Indirect Fluorescent Antibody Technique for Toxoplasma antibodies. The prevalence of antibodies to Toxoplasma gondii in Peru is high (38%). In subjects from Lima (150 m above sea level), representing the coastal region, a prevalence of 42% was found in subjects from Huancayo (3400 m) and Cerro de Pasco (4300 m) representing the Andean region, the prevalence was 9% and in Sivia, Tingo Maria and Pucallpa, located in the jungle region (under 660 m) the prevalence was 47%. The distribution pattern of the IFA titers was also found to be different for each of the 3 regions. The highest titers were found in the jungle region (between 1/1024 and 1/8192), while in the Andean region no titers were found higher than 1/16. There was no indication that sex was an important factor in the prevalence of Toxoplasma antibodies. This study of the relationship between the prevalence of Toxoplasma antibodies and geography indicates an inverse correlation with altitude. Other factors affecting the prevalence of Toxoplasmosis, such as climate and ecology, are also discussed. (19 references).