Cells of osteogenic sarcoma line 791T were treated in vitro with a selectively cytotoxic methotrexate-human serum albumin-monoclonal antibody conjugate at concentrations which were toxic but allowed the "escape" of a small number of tumour cell colonies (less than 0.3% compared with controls). These colonies were propagated as clones in order to test their expression of the monoclonal antibody ( 791T /36)-defined antigen and their resistance to methotrexate (MTX) by comparison with parental cells. Most of the conjugate-treated clones were incapable of prolonged growth and died out, in contrast to untreated 791T clones which virtually always grow progressively. Only four treated clones grew at rates comparable with the parental line. Flow cytofluorometric analysis indicated that the surviving clones expressed normal or enhanced amounts of 791T /36-defined antigen and clonogenic assays demonstrated that they were sensitive to cytotoxicity by MTX. As could be predicted from these results, further exposure to the conjugate inhibited growth of the clones at doses comparable with those active against parental 791T cells. It is concluded that tumour cell clones emerging after exposure to a toxic concentration of a drug-antibody conjugate are not necessarily modified resistant clones, but may have severely impaired long-term growth potential or be susceptible to further contact with the same conjugate.