A method was developed which involved electroimmunoassay and crossed immunoelectrophoresis of subtilopeptidase A (EC 18.104.22.168). Initial trials with unfractionated antiserum were not successful and interaction of the enzyme with non-immunoglobulin serum components were shown to be the cause of the failures. Quantitative immunoelectrophoresis was possible when purified immunoglobulins were used. A pH of 6.5 (lower than the usual pH 8.6) was necessary to obtain a proper baseline definition. Subtilopeptidase A was confirmed as a multiple isoenzyme system. Qualitative inter-batch variations were detected. Di-isopropyl phosphorofluoridate inhibition altered the electrophoretic pattern, but no loss of antigenic determinants was observed.