Synopsis Families with more than one member affected by schizophrenia were identified and their members were interviewed. Four standardized diagnostic definitions (PSE, DSM-III, ICD-9, Feighner) were applied to all subjects who were classified as schizophrenic or not schizophrenic according to each definition. Non-schizophrenic psychiatric disorders which have been shown to be familially associated with schizophrenia were also identified. Twenty blood markers were ascertained for all subjects and evidence of co-segregation with schizophrenia was sought. No selective segregation was found and therefore there was no evidence suggesting linkage or supporting a monogenic theory of transmission of susceptibility to schizophrenia.