Immunotoxins (ITs) appear to vary considerably in their killing efficiency towards antigen positive cells. In order to unravel the mechanisms underlying these differences, the parameters responsible for these variations were studied. The efficacy of the monoclonal antibodies (MoAbs) WT32 (CD3), OKT4 (CD4), T101 (CD5), WT1 (CD7) and WT82 (CD8) conjugated to ricin A-chain was expressed by the extent of protein synthesis inhibition of four leukaemic T cell lines (CEM, GH1, Jurkat and HPB-ALL). Large differences in cytotoxicity were observed. Efficient inhibition of protein synthesis was seen with anti-CD3 IT, anti-CD5 IT and anti-CD7 IT. In these cases the cytotoxicity was related to the antigen density on the target cell membrane. Anti-CD4 IT inhibited poorly and anti-CD8 IT was ineffective, even on cell lines with a high expression of the corresponding antigen. When antigen density and cytotoxicity were plotted for all CD antigens, no correlation could be found. Subsequently, internalization was studied with 125Iodine-labelled antibodies. Anti-CD7 showed the fastest internalization rate, followed by anti-CD3 and anti-CD5. Anti-CD4 and anti-CD8 antibodies were respectively moderately and hardly internalized. When the absolute amount of internalized MoAb was calculated, a highly significant correlation with cytotoxicity was found. We conclude that the degree of antigen expression is not so important as the absolute amount of antibody internalized in predicting the efficacy of ITs.