Symptomatic thrombocytopenia is reported in a patient with sickle cell-beta thalassemia. Recovery following removal of a large, congested, and multiply infarcted spleen documented its hypersplenic origin. Three other noteworthy features of sickle cell-beta thalassemia were demonstrated by a study of this case. First, recognition was aided by isoelectric focusing in polyacrylamide gel, a new technique for hemoglobin analysis. Second, the patient had an unusually low percentage of sickle hemoglobin, yet moderately severe symptomatology. Finally, family study revealed no genetic recombination in the six offspring of the propositus and of her similarly affected sister, further documenting the close linkage of the locus for beta thalassemia and the structural locus for the beta chain.