The interactions between platelets and dialysis membranes were studied prospectively in 10 patients undergoing long-term stable dialysis. Transient but significant thrombocytopenia and platelet activation were found during dialysis with the commonly used cuprophane membrane. Platelet counts decreased from 231 +/- 21 X 10(3)/mm3 before dialysis to 127 +/- 28 X 10(3)/mm3 at 90 minutes following initiation of dialysis (p less than or equal to 0.007). Thromboxane B2, an index of platelet activation, also increased from a baseline level of 1.06 +/- 0.2 pg/10(6) platelets to 7.3 +/- 3.0 pg/10(6) platelets at 90 minutes (p less than or equal to 0.04). Cuprophane membranes were also shown to induce complement activation with C3a desArg, the stable derivative of C3 activation, showing a threefold increase from baseline 15 minutes after initiation of dialysis. In contrast, during dialysis with a non-complement-activating dialyzer membrane, polymethylmethacrylate, thrombocytopenia and platelet activation were not observed. These data suggest that platelet activation and thrombocytopenia during hemodialysis are associated with complement activation during hemodialysis in a manner similar to dialysis-associated neutropenia.