Randomly selected sera from 1169 patients with no history indicative of toxoplasmosis were examined by the indirect haemagglutination test (IHT) for Toxoplasma gondii antibodies. Twenty-one percent were positive with titres of 1:64 or greater. No significant difference was demonstrated between the sexes. The antibody rate increased with age and the highest geometric mean of reciprocal titres was observed between the ages of 40 and 59 years. Among 63 patients with a history of symptoms suggestive of toxoplasmosis, 26 had titres between 128 and 2048. The detection of active infection and the possible source of infection are discussed.