Previous work correlating 51chromium release with aggregate formation between neutrophils and target cells had indicated that unlike killing by lymphocytes, neutrophil-mediated antibody-dependent cellular cytotoxicity (NADCC) in the guinea-pig against hepatoma cells required that three or more neutrophils were simultaneously attached to the target cells. There was virtually no lysis when only one or two neutrophils were attached. This raised the possibility that the target cells could repair lesions inflicted by one or two cells and this was investigated using inhibitors of macromolecular synthesis. Actinomycin D, cycloheximide, puromycin and adriamycin had no effect--militating against a repair mechanism. Surprisingly, mitomycin C led to an exacerbation of NADCC. Chlorambucil, which like mitomycin C is an alkylating agent, had a similar effect though less marked. One explanation of this phenomenon which seemed likely was that the drugs were affecting the cell cycle of the target cells, i.e. that being alkylating agents they might be holding the target cells in a susceptible phase of the cycle. Experiments in which the target cells alone were preincubated with mitomycin C for varying periods of time (2-18 h) before the start of the assay, did not confirm this. However a thirty minute preincubation with mitomycin C of either the neutrophils or neutrophils plus the target cells resulted in increased cytolysis. The results indicate that the effect of mitomycin C in exacerbating NADCC is primarily on the neutrophil.