The effect of leukocytes and erythrocytes on metabolic and functional changes was studied in 70 platelet concentrates kept for 72 hours at 22 degrees C in plastic PVC bags. On separating the platelet-rich plasma, a portion of buffy coat (i.e. a layer of leukocytes and erythrocytes) was added to one of the two series of platelet concentrates. These platelet concentrates did not differ from platelet concentrates prepared by the usual method, either by the platelet volume or count (p less than 0.05). The mean leukocyte count was 2.42 x 10(9)/L and 21.2 x 10(9)/L in the control and study series, respectively. After a 72 hour storage, a statistically significant difference (p less than 0.05) in the increase of lactate and ammonium concentrations, and a decrease in the glucose concentration and plasma pH were observed in platelet concentrates with a higher leukocyte count. The metabolic changes were accompanied by a statistically significant fall in the hypotonic shock (p less than 0.05). The results obtained in this study indicate that leukocytes in platelet concentrates elicit an adverse effect upon the platelet metabolism and function in vitro. In order to maintain the pH value at 6.0 during a 72 hour storage, the leukocyte count in platelet concentrates should not exceed 15.6 x 10(9)/L.