We analysed the impact of the area of the dialysis membrane and of its content in hydroxyl moieties on the magnitude of haemodialysis-induced complement activation and leucopenia. First, in five patients successively treated with cellulose acetate membranes of different areas, we found that increase of the area results in an increase in complement activation determined by C3a levels before and at 15 min of dialysis. The levels of leucopenia were similarly affected, and a significant correlation was found between complement activation and leucopenia (r = 0.89, P less than 0.05). When we compared the biocompatibility characteristics of a dialyser made of saponified cellulose ester with those of two dialysers made of cellulose acetate, we found that index of complement activation (18.1 +/- 2 vs 9.9 +/- 1.1 and 11.5 +/- 1.1, P less than 0.01) as well as index of leucopenia (69 +/- 4 vs 40 +/- 2 and 37 +/- 3, P less than 0.001) were significantly higher on the saponified cellulose ester dialysers suggesting that hydroxyl groups of cellulosic membranes play an important role in the pathophysiology of dialysis-induced changes.