Peripheral blood mononuclear cells, preincubated in ampicillin-containing media, exhibited a markedly decreased antibody-dependent (ADCC) and lectin-induced (LICC) cellular cytotoxicity against L1210 target cells in 9 out of 20 and 7 out of 13 donors, respectively. Additional experiments revealed that ampicillin effect on ADCC is based upon the blocking of the Fcgamma receptor resynthesis, in the case of their damage occurring in the process of cell separation.