Two families are described in whom there is an inherited defect of platelet aggregation in response to ADP. This consists of a normal response to low concentrations but a reduced response to concentrations adequate to produce complete, irreversible aggregation in normal subjects. Ten times the usual concentration is required to produce such maximal aggregation. Aggregation in response to collagen is normal as is release of ADP in response to collagen. This anomalous behaviour is not due to a plasma ADPase. This defective behaviour can be duplicated by suitable manipulation of normal platelets and may not occur in vivo. The existence of this anomalous behaviour contradicts the hypothesis that collagen-induced aggregation is mediated by ADP released from platelets. This disorder has been named ‘Pearson-Stoba anomaly’.