The presence of antibody isotypes (IgG, IgA and IgM) to streptolysin O was determined by dot ELISA in 222 serum samples from patients with different levels of anti-streptolysin O (SLO) antibodies as measured by the neutralizing assay (NA), from patients with diseases not related to nonsuppurative complications of Streptococcus pyogenes infection, and from clinically healthy individuals. Immunoglobulin G antibodies were found in 72% of sera from patients with SLO antibodies higher than 333 Todd units (TU), and IgA antibodies were also detected in 53%, but no IgM antibodies were demonstrable. High copositivity (0.94), conegativity (0.97), and positive (0.96) and negative (0.96) predictive values were observed when IgG and IgA findings were combined. The dot ELISA gave highly reproducible results. The present data suggest that the assay may be of practical value for routine detection of SLO antibodies when employed with an anti-human immunoglobulin light chain peroxidase conjugate.