Using two-dimensional electrophoresis (IEF, pH 3.5-0.0 and PAGE, 11.5% T, pH 7.9) the caprine plasma proteinase inhibitors were classified into six distinct classes, designated PIA, PIB, PIC, PID, PIE and PIF. Differentiation of the six inhibitors was based on electrophoretic criteria, their abilities to inhibit bovine trypsin and chymotrypsin and their cross reactions with antisera to human α1-antitrypsin and α1-antichymotryspin. Polymorphic variants were identified for five of the protein systems (PIA, PIB, PIC, PID and PIE) and the electrophoretic data indicated that the variants were controlled by allelic genes. PIF proteins were poorly resolved and invariant. Treatment of selected plasmas with neuraminidase demonstrated that the microheterogeneity observed in the PIA, PIB, PIC and PID proteins was attributable to sialic acid additions. The inhibitory activities of all six caprine proteinase inhibitors were unaffected by chemical oxidation with chloramine-T.