The microheterogeneity of alpha 1-acid glycoprotein (AGP) has been studied in the sera of 48 patients with rheumatoid arthritis and of 12 healthy individuals. For each rheumatoid patient the disease activity has been assessed and each patient has been assigned to one of four activity grades: I, inactive; II, mildly active; III, moderately active; and IV, severe. Concanavalin A-affinity electrophoresis revealed three microheterogeneity variants of AGP: non-reactive with Con A, weakly reactive with Con A and strongly reactive with Con A. The relative amounts of AGP-variants observed in the healthy donors were similar to those observed in the patients with activity grade I, but differed significantly from patients with grades II, III and IV. The differences between the AGP-patterns of patients with activity grades II, III and IV were also statistically significant.