Cerebrospinal fluid from 13 of 38 patients with schizophrenia had a cytopathic effect in cultures of MRC5 cells. The cytopathic agent passed a 100 nm but not a 50 nm filter and was unaffected by heat at 56 degrees C for 30 min, treatment with chloroform, or the presence in cultures of bromodeoxyuridine. The agent could not be propagated serially in a satisfactory manner but its properties were those of a virus. A similar cytopathic effect was induced by cerebrospinal fluid from 8 of 11 patients with serious or chronic nervous system disease but only 1 of 25 patients with surgical or general medical conditions.