We examined whether different human colorectal carcinomas (HCC) and cells populating a single neoplasm exhibit a heterogeneous response to the cytostatic and cytolytic effects of recombinant human cytokines used clinically for cancer therapy. The effects of interferon-alpha (IFN-alpha) hybrid BBDD, interferon-gamma (IFN-gamma), tumor necrosis factor (TNF), interleukin-1 (IL-1), and the chemotherapeutic drug 5-fluorouracil (used as a positive control for cytostasis) were examined in six recently established HCC cell lines. One cell line was sensitive to the cytostatic and cytotoxic effects of IFN-alpha BBDD, whereas five cell lines were sensitive to IFN-gamma. Tumor cell sensitivity to IFN-alpha BBDD was independent of sensitivity to IFN-gamma. Only one cell line was somewhat sensitive to the cytostatic and cytotoxic effect of TNF, and none was sensitive to IL-1. The heterogeneous response to cytokines by a single neoplasm was demonstrated by analysis of multiple clones. Clonal populations exhibited different levels of susceptibility to cytostatic effects mediated by cytokines. The combination of IFN-gamma with IFN-alpha BBDD or with TNF produced additive or even synergistic cytostasis in HCC lines sensitive to the cytokines but not in cell lines that were resistant to either one or both agents. Collectively, these studies demonstrate intra- and intertumoral heterogeneity in sensitivity to recombinant cytokines, a finding that should receive consideration for clinical application.