The microheterogeneity of alpha 1 acid glycoprotein (AGP) was studied using affinity immunoelectrophoresis with concanavalin A (Con A) in serum samples of 43 patients with early rheumatoid arthritis (RA) without clinical features of intercurrent infection. The results were expressed as reactivity coefficients. Disease activity was measured by clinical (Lansbury's joint index, Mallya-Mace activity score) and laboratory (erythrocyte sedimentation rate, levels of soluble interleukin-2 receptor, C reactive protein, and AGP) indices. In contrast with previous reports, suggesting a decrease in AGP-Con A reactivity in patients with RA, high values of AGP reactivity coefficients were found in patients with disease of short duration, which were similar to those found in patients with acute bacterial infections. Conversely, normal or decreased values of AGP reactivity coefficients were found in patients with disease of longer duration. Regression analysis showed a significant relation between AGP reactivity coefficients and disease duration (multiplicative model). No other indices examined were significantly related to disease duration. These results, taken together with previous findings suggesting that cytokines control the glycosylation of acute phase proteins, indicate that differences in the microheterogeneity of AGP in early and longstanding RA reflect differences in cytokine action at different stages of the disease.