We investigated the contribution of albumin to serum fructosamine activity. On gel exclusion chromatography, pooled diabetic and pooled non-diabetic sera before and after incubation with [14C]glucose differed in fructosamine activity and radioactivity mainly in the albumin-containing fractions. We confirmed the major contribution of glycated albumin to serum fructosamine concentration using affinity chromatography on Blue Sepharose. The importance of high molecular mass species was further demonstrated by the recovery of reducing activity following dialysis and charcoal treatment. Fructosamine measurements in diabetic patients were not correlated with serum albumin concentration in individuals with urinary albumin less than 1 g/l. We conclude that fructosamine activity is a convenient measure of glycosylprotein concentration and that the index responds mainly to glycation of albumin. However, routine correction for serum albumin concentration is inappropriate.