The effect of radiographic contrast media on proteins of the complement system in vitro was investigated using protein level measurement and crossed immunoelectrophoresis. Despite the 20% loss of total serum hemolytic activity (CH50) induced by sodium meglumine ioxitalamate (0.5 M), no significant changes in C3c, C3N or B were detected by radial immunodiffusion. These contrast media did not cause electrophoretic conversion of C3, as assessed by the absence of split product, and the small degree of spontaneous C3 conversion (loss of 20% C3N antigen and the presence of two peaks with C3c antigen) which occurred when serum was incubated in polypropylene tubes, was inhibited by the contrast media. The effects of sodium meglumine ioxitalamate and zymosan in vitro on complement activation were compared.