Treatment schedules were investigated for in vivo induction of lymphokine-activated killer cells in the rat liver. Treatment of rats with continuous sys- temic or regional infusion of recombinant human interleukin-2 with a dose of 4 to 8 x lo4 U/day during 7 days, resulted in an increase in number of large granular lymphocytes or pit cells in the liver up to 43 times normal. Kupffer cells, nongranular lymphocytes, monocytes and neutrophils also increased in number, but with a maximal fivefold increase this was much less pronounced than for large granular lymphocytes. Kupffer cells showed morphological signs of activation and were frequently seen in mitosis. Frequent mitoses were also observed for large granular lymphocytes, but not for other leukocytes. This indicates that the effect of interleukin-2 treatment on hepatic (sinusoidal) cells was primarily directed to large granular lymphocytes and Kupffer cells. The large granular lymphocyte accumulation occurred mainly intrasinusoidally, but they were also frequently observed in the space of Disse where they are not found in control rats. This may be explained partly by the observed damage or gap in the endothelial lining. The intrasinusoidal large granular lymphocytes adhered to the endothelium and to Kupffer cells. Higher responses, for all cell types, were found when interleukin-2 was administered regionally, that is, through the hepatic artery rather than through the systemic route (jugular vein), although the dif- ferences were not statistically significant. Doses below 4 x lo4 U/day did not result in significant increases of large granular lymphocytes in the liver.