Monoclonal antibodies (MAbs) N-59 and RU-1 were produced by immunisation of mice with streptokinase secreted by Streptococcus group A, type 12, strain A374 isolated from a patient with post-streptococcal glomerulonephritis (PSGN) and were characterised by Western blot analysis. MAb N-59 recognised antigenic determinants shared by both nephritis strain-associated streptokinase (NSA-SKase) and streptokinase of Streptococcus group C (C-SKase); MAb RU-1 reacted only with NSA-SKase. All nephritis-associated group A streptococcal strains tested reacted with MAb N-59; 87.5% of these strains reacted with MAb RU-1. MAb N-59 reacted with SKase produced by group G streptococcal strains isolated from patients with PSGN, and MAb RU-1 recognised SKase in two out of three of these strains.