A psychopharmacogenetic strategy was used to investigate a genetic heterogeneity model of schizophrenia. This model consisted of various genetic subtypes represented by patients classified hypothetically according to the types and genealogical (Mendelian) patterns of illnesses in first-degree relatives. The effect of neuroleptics on these subtypes (drug x genetic subtype interactions) were tested for evidence of post-treatment responses which discriminated between them. The findings revealed that schizophrenics who had depressed relatives tended to exhibit (1) depression and more severe pseudoparkinsonism irrespective of types of neuroleptics, and (2) greater remission of paranoid-hostility symptoms when treated with neuroleptics of the aliphatic-piperadine type. Schizophrenics who had schizophrenic relatives failed to show these responses. Interpretation of these findings emphasized the recognition of these responses as arising from neuroleptic-induced alterations of defective neurologic-neurochemical systems underlying this subtype and as "pharmacogenetic criteria" by which it can be discriminated.