A quantal assay, based on syncytium formation in the human T cell leukaemia-derived C8166 cell line, was used to determine the kinetics of human immuno-deficiency virus type 1 (HIV-1) strain IIIB neutralization. Three rat monoclonal antibodies (MAbs) were used, under physiological conditions of temperature and antibody concentration. MAb ICR39.3b (IgG2b) neutralized virus with no lag period while the other two MAbs, ICR39.13g (IgG2b) and ICR41.1i (IgG2a), neutralized with lag periods of 5 min and 15 min respectively. It was calculated that the latter two MAbs mediated neutralization by about two and three molecules of IgG per virion respectively. The highest neutralization rate constant (for MAb ICR 41.1i) was over 300-fold less than that of MAbs specific for the haemagglutinin of the enveloped influenza virus type A and for poliovirus type 1.