In this work the resistance of peroxisome-proliferated hepatocytes to hydrogen peroxide (H2O2) has been studied. The question has been raised as to whether this resistance is a response to cytotoxicity. In an initial series of experiments, hepatocytes were isolated from rats that had been treated with nafenopin (NAF-hepatocytes). Isolated cells were exposed to a H2O2-generating system or to H2O2 in pulses. The ability to attach to collagen was used as a toxicological endpoint. Loss of attachment was found to be correlated to glutathione (GSH) depletion, and NAF-hepatocytes were more resistant to GSH depletion and to loss of attachment induced by H2O2 than were control hepatocytes. NAF-hepatocytes were not resistant to hydroquinone or to adriamycin. It was also indicated that this resistance was related to an altered metabolism of H2O2, less dependent on GSH. In a second series of experiments, hepatocytes from altered hepatic foci-bearing rats, treated with nafenopin or di(2-ethylhexyl)phthalate (DEHP), were used. This model was used in an attempt to monitor the development of resistance in different subpopulations of hepatocytes. It was found that the majority of hepatocytes developed resistance towards H2O2, and that, for example, foci marker-positive hepatocytes were as resistant as marker-negative cells. In control experiments with this model, it was found that marker-positive cells were more resistant towards diethyl maleate (DEM) or phorone than were marker-negative cells. In addition to demonstrating the validity of the model, these control experiments indicate an increased steady-state level of H2O2 in cells from peroxisome proliferator-treated rats. Other control experiments suggested that a low GSH-peroxidase activity protected from, rather than aggravated, the effect of peroxisome proliferation on marker-negative and GSH-depleted cells. It is concluded that H2O2 metabolism may affect the function of collagen receptors, but that a shift in H2O2 metabolism, so that it becomes less dependent on GSH, conferred resistance to this effect. The apparent non-focal induction of resistance to peroxisome proliferators, as opposed to the focal induction of resistance induced by most liver carcinogens, may explain the lack of development of gamma-glutamyltranspeptidase-positive foci in peroxisome proliferator-treated rats.