In studies of hepatitis in an endemic zone in Costa Rica, 103 patients were examined for antibodies against hepatitts A by the immune-adherence assay, for hepatitis B antigen and its antibody by radioimmunoassay and passive hemagglutination, respectively, and for antibodies against cytomegalovirus by complement fixation. Twelve cases were encountered in which both Type A and Type B hepatitis could be excluded on the basis of serologic testing. In all but one of these 12 patients, cytomegalovirus infection was also excluded. The patients had not had blood transfusions and available evidence pointed to person-to-person transmission. The illness in these patients was evidently neither hepatitis A nor hepatitis B and qualifies for consideration as the still hypothetical third type of hepatitis ("C"?).