The heterogeneity of tumour antigen expression, the differential sensitivity of individual cells to drugs and the use drug–antibody immunoconjugates with limited potency can limit the antitumour effects of immunoconjugate therapy. In this study we have used two different antibodies linked to two different drugs Melphalan (Mel) and Idarubicin (Ida), each with a different site action, to evaluate the potential of using cocktails of immunoconjugates. A series of drug combinations were screened for their synergistic activity in vitro using the inhibition of [3H]-thyrmidine uptake by E3 cells, and constructing isobolagrams: Mel plus Ida was the only combination found to be synergistic in vitro and this synergism extended to the drugs after conjugation to antibodies. In addition, in vivo studies in mice bearing E3 tumours showed that synergy between both free drugs and between Ida-anti-Ly-2.1 and N-AcMEL-anti-Ly-3.1 immunoconjugates was time dependent, requiring treatment with Ida or Ida-MoAb conjugates prior to the addition of the second melphalan containing immunoconjugate. The use of two different antibodies, anti-Ly-2.1 and anti-Ly-3.1 against E3 (Ly-2.1+ve, Ly-3.1+ve) gave greater synergy in vitro compared to using only one antibody. Again a cocktail of two antibody immunoconjugates provided significantly greater antitumour efficacy when given to tumour bearing mice, provided that the Ida-anti-Ly-2.1 was given 24 h before injection of N-AcMEL-anti-Ly-3.1. The enhanced antitumour effect was not observed when the immunoconjugates were given simultaneously, or if the same antibody was used in each conjugate. Of importance was the finding that although the anti-tumour effect was synergistic, there was no increase in toxicity noted. The increased therapeutic index observed by using a double cocktail (2 antibodies + 2 drugs) could have major implications for immunoconjugate therapy.