Circulating antigens were investigated, by immunodiffusion, in sera and urines from Schistosoma mansoni infected patients. The occurrence of a thermostable parasitic urinary antigen, called M, was demonstrated in 80% of infected patients. The incidence of M component was proportional to the fecal egg count and was inversely proportional to the age of the patient. Specific anti-M precipitating antibodies were demonstrated in 41% of sera from infected patients. A statistical correlation was found between the presence of M antigen and the level of total and anti-S. mansoni IgE, and also with the 24-hour intradermal test response with S. mansoni antigen. The specificity, the origin, and the function in the immune response of the M antigen are discussed.