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Background: While there is evidence that some cases of schizophrenia may be associated with microbial infections, the role of microbial agents has not been investigated in people with emerging psychosis. Methods: Participants were 105 help seeking ultra-high risk individuals. Psychiatric measures included the Brief Psychiatric Rating Scale and the Scale for the Assessment of Negative Symptoms. Serum IgG antibodies against human herpesviruses and Toxoplasma gondii were determined using immunoassay methods. Multiple linear regression with adjustment for age and sex was applied to test associations between serum antibodies and psychiatric measures. Results: Higher levels of serum IgG antibodies against Toxoplosma gondii in Toxoplasma-positive individuals were significantly associated with more severe positive psychotic symptoms. No significant association was observed between antibody levels and psychiatric measures in individuals positive for human herpesviruses. Conclusions: In some individuals infection with Toxoplasma gondii may be an environmental factor contributing to the manifestation of positive psychotic symptoms.