The platelet aggregation responses induced by adenosine diphosphate (ADP) and collagen were characterized by parameters defining rate and extent of reaction. Values were compared on the basis of platelet concentration, time elapsed between sample collection and test performance, and sex of donors. Rate and extent of aggregation varied as a function of platelet concentration. Maximal responses were obtained for platelet concentrations greater than 100,000/microliter. Results from samples processed after 90 min of incubation at room temperature were consistently and significantly lower than those obtained from samples processed immediately. ADP-induced aggregation responses elicited by samples from male and female normal donors differed significantly. This indicates that comparisons between normal and patient results should be made with sex-matched individuals to avoid erroneous interpretations.