The authors studied the MLC blocking effect of anti-DRW anti-sera. These sera were harvested from multiparous women and their anti-DRW activities were defined (after exhaustive platelets absorption) on a panel of 73 B-lymphocytes. All the six tested sera were strongly inhibitor until dilution greater than 1/500 when they were present in the MLC for all the culture period but without specificity for their relevant target cell. The MLC inhibitory activity was still found in IgG fraction. A significant specific inhibition could be found only when the cultured cells were independently preincubated with the corresponding anti-DRW serum prior culture. In this last case inhibition occured when the target cell (bearing the DRW antigen recognized by the studied sera) was used as the stimulator cell in MLC. A decrease of the inhibition was found when the anti-DRW was absorbed with its relevant target cell but also with non-target cell, as assessed by B-cell cytotoxicity, suggesting that non cytotoxic antibodies directed against "public" determinant are involved in MLC inhibition.