In hemodialysis using 3 types of dialysis membrane materials [regenerated cellulose (RC), cellulose triacetate (CTA), and polysulfone (PS)], activation of the complement, reduction of white blood cells, and variation of vitronectin (VN) were observed. RC membrane caused a significant reduction of white blood cells and elevations of Bb and soluble membrane attack complex (S-MAC), indicating a strong activation of the alternative complement pathway. Especially, the increase of S-MAC persisted for a long time during hemodialysis. Because reduction of VN was transient, it was assumed that the S-MAC escaping removal by VN receptors might have persisted in the circulation. These findings suggested that S-MAC would become useful as an index for evaluating biocompatibility of various artificial organs including dialysis membranes.