The most common platelet disorder which is responsible for abnormal bleeding is thrombocytopenia. In other cases, however, the platelets are normal in number, but appear to be defective in function. Of the many platelet factors, factor 3, or the thromboplastic factor, has been studied most extensively in various conditions. Deficiencies in factor 3 have been reported in uremia, cirrhosis, leukemia, myeloprolif erative disorders, lupus erythematosus, thrombocytopenic purpura, and other disorders. This "qualitative" platelet defect may contribute to the bleeding tendency in these condtions. The results obtained in specific disorders by various investigators have differed,1,2 but the variability no doubt reflects differences in the technique used to measure factor 3. Recently, we have reported the use of a quantitative assay for factor 3, using the thromboplastin generation test (TGT).3 We used a dilute (0.033%) suspension of washed platelets, prepared volumetrically in graduated, thin-stem centrifuge tubes. By this method, we