An interest in hemodialysis-related patient symptomatology dating back to the beginnings of maintenance hemodialysis (HD) therapy prompted our investigations to determine the etiology of such discomfort. Since the discovery by Craddock et al (1) of the activation of the alternative complement pathway by hemodialyzer membrane, our efforts toward defining the relationship of symptoms to complement activation have been done in collaboration with the Minnesota group. Results of an earlier blinded study, (2) and continuing investigations of HD membranes provide data which support the contention that complement activation, although not necessarily etiologic in the symptoms related to dialysis, serves as an indicator of membrane compatibility. These newer data reveal C3a rises in vivo and in vitro by a Japanese processed cuprammonium membrane for dialysis to be similar to cuprophane. In vivo C3a elevation using Travenol CA-110 hemodialyzers of cellulose acetate are significantly lower and are similar to earlier results obtained by cellulose acetate of CD manufacture.