Many human tumor cell lines produce alkaline phosphatases (APs) whose expression is normally not associated with the tissue where the tumor originated. The control of high level AP expression in three cloned human tumor cell lines was examined after fusion to murine fibroblast cell lines. The human tumor cell lines studied were: HeLa 71.5, producing term placental AP, D98.8, producing fetal intestinal AP and Hs578T.12, producing a liver-like AP. After fusion to the murine fibroblast cell lines IT22 or CL1D and propagation of the hybrids, the presence of human chromosome markers and the levels of AP were determined. Hybrids of either murine line fused to HeLa 71.5 or D98.8 cells never gave rise to proliferating clones with significant levels of their respective APs, even when several hundred hybrids were examined, in situ, at early and later stages of proliferation. Consequently, the abundant AP production normally found in HeLa 71.5 and D98.8 cells may require products from a number of chromosomes or the mouse cytoplasm is able to extinguish AP expression. Hybrids formed between the Hs587T cell line and either murine line continued to express liver-like AP. This expression was maintained in HAT selection medium and eliminated with bromodeoxyuridine counter selection. This result suggests that the control of liver-like AP in human tumors differs from that of the full-term placenta of fetal intestine.