1. A family study was carried out using a putative biological vulnerability trait in families of schizophrenics and schizoaffective indexprobands to investigate, if the clinical phenotype and a biological marker for schizophrenia are cosegregating within families. 2. The binding capacity of the dopamine antagonist spiperone to mononuclear cells was investigated in 21 indexprobands and a total of 147 first and second degree relatives. 3. Increased binding capacity could be found in 17 indexprobands and in their affected relatives, independently from clinical diagnosis and in 22% of their normal relatives. 4. No increased binding capacity was found in 4 indexprobands and in their affected relatives and not n any of the unaffected relatives. These results indicate, that increased spiperone binding may cosegregate with the risk for functional psychoses and that families, loaded with psychiatric disturbances may be distinguished on a biological basis.