Clinical as well as laboratory studies have been employed to assess the complement activating potential of polycarbonate membrane hemodialyzers. Blood samples from a group of patients undergoing sequential maintenance hemodialysis with cuprophane, polyacrylonitrile and polycarbonate devices were evaluated to define plasma levels of C3a antigen and leukocyte counts during the initial phases of hemodialysis. While polyacrylonitrile dialyzers did not activate complement to a significant extent, we did observe transient elevations in the plasma concentration of C3a and corresponding diminutions in the granulocyte counts of patients dialyzed with both cuprophane and polycarbonate dialyzers. However, polycarbonate devices appeared to activate complement to a lesser degree than cellulosic dialyzers. Laboratory evaluation of these three different types of dialyzers also provided evidence that polycarbonate membranes did not appear to activate human complement as readily as cuprophane. These observations suggest that polycarbonate membranes display complement-related biocompatibility properties that are intermediate between those of cuprophane and polyacrylonitrile.