ABSTRACT: The detection of early pregnancy factor (EPF) is a controversial subject. The findings of independent groups attempting to confirm the phenomenon of EPF have been quite variable. The present article gives details of an improved rosette inhibition assay using a monoclonal antibody (anti-human-Lyt-3), instead of an anti-lymphocyte serum (ALS), in order to increase reproducibility. Results are calculated for the first time in terms of absolute monoclonal anti-Lyt-3 concentrations. It is suggested that results be expressed as the Rosette Inhibition Antibody Concentration (RIAC) or the more accurate extrapolated 25% Rosette Inhibition Antibody Concentration (25% RIAC), in place of the previously used Rosette Inhibition Titer (RIT), when monoclonal antibodies are used. Following a 1.5 h incubation period the mean 25% RIAC for pregnant serum was 15.9 ± 4.0 ng/ml and for control serum 48.0 ±12.7 ng anti-Lyt-3/ml. The improved method described should enable different groups to establish the test and compare results.