Five abnormal prothrombin time pooled patient plasma samples and seven commercially prepared abnormal prothrombin time control plasmas were compared. A fibrometer was used to run prothrombin time tests from 8 to 51 days on individual aliquots of the pooled patient plasmas and individual vials of the commercial products. Pooled patient plasmas exhibited greater reproducibility with generally lower coefficients of variation than did the commercial products, with the added advantage that they reflected the patient population more directly. Almost all samples displayed a significant rise in mean clotting times during the testing period. However, the reasons for this rise are not entirely clear.