Twenty-hour donor platelet recoveries of single treatments of PRP. (A) All 5 recipients of γ-I platelets became platelet refractory with very poor recoveries even with their first donor transfusion. There were no differences between means of pretreatment (47% ± 11%) and posttreatment (49% ± 7%) autologous platelet recoveries (P = .66) nor between pretreated and treated (44% ± 9%) autologous platelet recoveries (P = .42). (B) Six of 7 recipients (86%) of MPR became refractory on treatment. One of these refractory dogs developed posttransfusion purpura after the fifth transfusion (her second refractory transfusion) when the recipient’s platelet count fell from 261 000 per microliter at the end of week 4 to 3000 per microliter the next week, and the dog was killed. The dog had high levels of antibodies to both her donor’s platelets (2.2 × autologous control sera) and lymphocytes (1.7 × autologous control sera). No autoantibodies were sought because the dog’s platelet count was too low. There were no differences between means of pretreatment (47% ± 5%) versus posttreatment (49% ± 11%) autologous platelet recoveries (P = .76), but there was a significant difference between pretreated and treated (21% ± 7%) autologous platelet recoveries (P < .001). PTP, posttransfusion purpura.