In vitro aggregation of the platelets from four patients with refractory anemia and two patients with acute myelomonocyctic leukemia revealed distinctive abnormalities. In five patients, there was deficient or minimal aggregation with adenosine diphosphate (ADP), epinephrine, or collagen and only one wave of aggregation could be elicited with ADP at any concentration. Ultrastructural studies revealed numerous isolated platelets, small aggregates with few platelet pseudopods, and the presence of a characteristic type of aggregate with heterogeneous platelet composition combining features of both the primary and the secondary waves of aggregation. These "mixed aggregates" were particularly abundant in the four patients who had refractory anemia and may constitute the structural basis of the single wave of aggregation observed.