Leukemia inhibitory factor (LIF) was tested using three established acute myelocytic leukemia (AML) cell lines. In growth assays in two of the three lines, we found that the addition of LIF increased the doubling time of the clonogenic population but not the total population as assessed by nucleated cell counts. Similarly, tritiated thymidine uptake into total AML populations was not affected by LIF, but the percentage of clonogenic cells killed by exposure to high specific activity 3HTdR was reduced in LIF-treated cultures compared to controls. We interpret these results to indicate that LIF prolongs the cell cycle of stem cells in some AML lines, possibly by increasing the time spent in the G2-M-G1 parts of the cycle. Consistent with this interpretation, we observed a decrease in ara-C sensitivity in LiF treated cultures. Variable results were obtained when freshly obtained AML blasts were exposed to LIF.