The mechanism of action of an abrin 9.2.27 antimelanoma antibody conjugate has been studied in 2 human melanoma cell lines, FEMX and LOX, which differ in sensitivity to the immunotoxin (IT) and to native abrin. The IT, which had been affinity purified before use to remove molecules with exposed gal-binding sites on the toxin B-chain, inhibited cellular protein synthesis at a faster rate in the L O X than in the FEMX cells despite the fact that the L O X cells express less specific antigen and bind less IT to the cell surface. Surface-bound abrin-IT, as well as surface-bound specific antibody, disappeared at equal rates from the cell surface of the 2 cell lines. After binding of labelled IT the disappearance of total cellassociated radioactivity, as well as the appearance of TCAprecipitable and TCA-soluble radioactivity in the incubation medium, occurred at faster rates in the LOX than in the FEMX cells. No free abrin or antibody B-chain complex could be detected in the medium or inside the cells. The results indicate that the different sensitivities of the melanoma cell lines reflect different abilities to process endocytosed IT and to translocate the active A-chain to the cytosol. Experiments carried out in the presence of lactose are interpreted to mean that the A-chain may be translocated to the cytosol by two mechanisms, one involving antigen-antibody interaction and one involving the 8-chain, and that the lectin binding site contributes to the B-chain-facilitated mechanism.