The aim of this study was to compare the effect of cyclosporin A (CsA) in inbred Lewis rats with published assessment of immunotoxicity in 'classical' outbred Wistar rats. A second purpose was to consider the contribution of a panel of in vitro assays in cell cultures when added to an immunotoxicity study in vivo. The in vivo effect of CsA was investigated in a 28-day subacute immunotoxicity study in male Lewis rats at three different concentrations: 1.25, 5 and 20 mg kg(-1). The highest dose of CsA exceeded the maximum tolerated dose. A drop in body, spleen and popliteal lymph node weight of exposed animals displayed symptoms of toxicity. At a high toxic dose, haematological changes showed a decrease in the leucocyte count and in the percentage of lymphocytes, and an increase in the percentage of polymorphonuclear leucocytes. The haematocrit was significantly dose-dependently suppressed in all rats exposed to CsA. A similar dose-dependent depression of the mean cell volume of erythrocytes was found in rats given high and middle doses of CsA. The phagocytic activity of polymorphonuclear leucocytes and monocytes also was significantly dose-dependently suppressed. No significant changes in primary antibody response to sheep erythrocytes or in vitro proliferative response of spleen lymphocytes to mitogens were found in those rats.A battery of in vitro cytotoxicity methods was selected for the evaluation of metabolic and functional activity of subcellular organelles (mitochondria, lysosomes) and for the detection of drug-induced superoxide-mediated damage in HeLa cells. This cell line was chosen because it has a lower activity of superoxide dismutase (SOD) than normal cells and is sufficiently sensitive for the detection of the induction of oxygen radicals. The in vitro results indicated a direct relationship between CsA cytotoxicity and a change in the mitochondrial enzyme activity, as well as an induction of superoxide production. The results of the study indicated that a combination of selected in vivo and in vitro methods is an inexpensive way to obtain more complex information on cell status affected by xenobiotics.