Microheterogeneity of acute phase proteins in the differentiation of polymyalgia rheumatica from polymyositis. We studied an alpha-1-acid glycoprotein (AGP) and an alpha-1-antichymotrypsin (ACHT) microheterogeneity in sera of patients with polymyalgia rheumatica (PMR), giant cell arteritis (GCA/PMR), polymyositis/dermatomyositis (PM/DM) and healthy individuals by affinity electrophoresis with concanavalin A (Con-A) as the ligand. Our results are expressed as reactivity coefficients. The mean of AGP reactivity coefficients (AG-RC +/- SD) in PMR (0.92 +/- 0.17) and GCA/PMR (0.91 +/- 0.12) were significantly lower compared with the mean AG-RC in patients with PM/DM (1.48 +/- 0.52) as well as in healthy individuals (1.34 +/- 0.9). Moreover, an additional microheterogeneous form of AGP was noted in patients with PM/DM. In parallel, we also found that the mean of ACHT reactivity coefficients (AC-RC +/- SD) were lower in patients with PMR (2.94 +/- 1.24) and GCA/PMR (1.66 +/- 0.16) compared with healthy individuals (3.92 +/- 1.17). The mean of AC-RC in patients with PM/DM (6.74 +/- 4.35) was significantly higher than in patients with PMR and GCA/PMR as well as in healthy individuals. Our results show that the changes in reactivity of AGP and ACHT with Con-A are useful diagnostic markers for the differentiation of PMR and GCA/PMR from PM/DM. 