1. A two compartment system, comprising two adjacent teflon chambers separated by a semi-permeable membrane, has been devised with which to investigate the generation of drug metabolites that are toxic to human cells in vitro. 2. Compartment A contained a drug-metabolising system (human liver microsomes +/- NADPH) and compartment B contained target cells (human mononuclear leucocytes). The semi-permeable membrane retained protein (m.w. greater than 10,000) but allowed equilibration (within 1 h) of drug and drug metabolites, during which time cells remained viable. 3. Incubation of dapsone (100 microM) with human microsomal protein (2 mg ml-1) and NADPH (1 mM) in compartment A caused cell death (8.7 +/- 1.8%) in compartment B, which was reduced significantly (P less than 0.05) by the addition of glutathione (500 microM). Dapsone in the absence of NADPH was not cytotoxic. 4. Chemical analysis showed the presence of dapsone hydroxylamine as the only stable metabolite in both compartment A (5.2 +/- 0.4% incubated drug) and compartment B (3.5 +/- 0.5%). 5. Irreversible binding of dapsone to cells was significantly (P less than 0.05) reduced by omission of NADPH (85 +/- 13 pmol/10(6) cells) or addition of glutathione (103 +/- 9) compared with control values (153 +/- 51).