In 1995-1997 the frequency of concurrent antibodies against Toxoplasma gondii and Borrelia burgdorferi was assessed in two groups of patients with different clinical symptoms. In 485 subjects the dermatological form (erythema migrans) of borreliosis was suspected, 877 subjects had clinical symptoms suggesting the nodular form of toxoplasmosis. The group of 485 subjects with suspected borreliosis comprised 199 (41%) subjects with the diagnosis confirmed by the serological finding and 286 (59%) subjects were serologically negative. During concurrent examination for toxoplasmosis a statistically significantly higher incidence of antitoxoplasmatic antibodies was recorded in patients with suspected borreliosis but without detected antibodies against Borrelia burgdorferi (29%) than in the group with both types of antibodies (11.5%), p < 0.001. The expected values of the incidence of antibodies against Toxoplasma gondii in the population (25%) were however not substantially surpassed by the actual values. In the group of 877 patients with suspected toxoplasmosis there were 114 subjects (13%) with acute toxoplasmosis, 300 subjects (34%) with anamnestic titres suggesting the chronic form of infection and 463 (53%) subjects where the infection with toxoplasmosis was not confirmed by serological tests. In the whole group, regardless whether the acute or chronic form of infection was involved, but also in the group without antitoxoplasmatic antibodies a significantly elevated number of subjects with antiborrelia antibodies (18-25%) was recorded as compared with the expected values of the incidence of antibodies against Borrelia burgdorferi in the population (10%), p < 0.001. In the discussion the authors deal with the possible causes of these phenomena.