For the first time, a screening procedure for antitumor monoclonal antibodies (MOABs) has been developed in which the ability of MOABs to mediate the indirect action of another immunotoxin is the primary criterion for selection of hybridomas for expansion and cloning. Use of the indirect immunotoxin makes it possible to screen MOABs for cytotoxic potential without the necessity of covalently coupling toxin to each individual MOAB. Hybridomas derived from spleens of immunized mice were screened for the synthesis of monoclonal antibodies able to participate in the delivery of a pharmacologically active, indirect immunotoxin conjugate to H69 lung cancer cells. The indirect immunotoxin conjugate was goat anti-mouse immunoglobulin disulfide linked to pokeweed antiviral protein. There was no correlation between the results of the screening with the indirect immunotoxin and an enzyme-linked immunosorbent assay screen for binding of MOAB to tumor cell membranes. An indirect immunotoxin prepared from Fab' fragments of goat anti-mouse immunoglobulin was also effective as an indirect immunotoxin conjugate. Each screening of more than 300 hybridomas was performed in 3 days. The indirect immunotoxin screen detected antitumor MOABs which were not detected by the conventional enzyme-linked immunosorbent assay method. A MOAB selected by the indirect immunotoxin screen was conjugated to pokeweed antiviral protein; the conjugate mediated immunocytotoxicity in a standard direct toxicity assay.