Antibody-mediated cytotoxicity against NIP-haptenized SK-BR–3 breast cancer cells was analyzed using NIP antibodies of human IgG1, IgG3, or IgA2 isotypes. As effector source, whole blood (▪), fresh human plasma (□), isolated MNC (▨), or PMN (▦) were compared. All three isotypes induced significant lysis with whole blood. Interestingly, IgA-dependent lysis was predominantly mediated by PMN, whereas IgG1- or IgG3- dependent killing resided mainly in the plasma fraction. Results from five experiments are presented as mean ± SEM of % specific lysis. Significant (P < .05) antibody-mediated lysis is marked by an asterisk (*).