Serum orosomucoid was compared with clinical activity, routine laboratory tests, intestinal protein loss, and albumin and IgG turnover in 22 cases of ulcerative colitis. Serum orosomucoid was well correlated with clinical activity, haemoglobin and leucocyte counts were not. A significant correlation was present between serum orosomucoid and intestinal protein loss (gastro-intestinal 59Fe-iron dextran clearance), serum albumin, fractional catabolic rates of albumin, and IgG and IgG synthesis rate. No correlation was found between serum orosomucoid and albumin synthesis rate or serum IgG. It is concluded that serum orosomucoid is a highly reliable indicator of disease activity in ulcerative colitis.