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The combination of differentiation-inducing agents with conventional antineoplastic drugs has been suggested as a potential new cancer therapeutic approach. We have assayed the cytotoxic effect of N-Methylformamide (NMF) as a differentiating agent combined with Paclitaxel, a novel antineoplastic agent, on cell survival of a human adenocarcinoma cell line HT29. The cell killing of this combination was evaluated by clonogenic assay and cell cycle perturbation was analyzed by flow cytometric methods. HT29 cells were exposed to graded doses of Paclitaxel (0.001-0.01-0.1-1-5 micrograms/ml) for 2, 4, 8, 16, 18 and 24 hours in order to determine its dose-time effect. Secondly, exponentially or non-growing HT29 cells were exposed to graded doses of Paclitaxel (0.001-5 micrograms/ml) for 18 hours at 37 degrees C, and in combination experiments the cells were pre- or posttreated with 1% NMF for 72 hours. The results demonstrated that only proliferating cells were responsive to Paclitaxel and that its cytotoxicity is strictly related to exposure time. The combination studies showed that only the Paclitaxel-->NMF sequence causes a powerful reduction in the surviving fraction of HT29 cells, whereas the reverse sequence had a protective effect on cell killing. The flow cytometry evaluation has indicated that synergism with NMF in HT29 cells was observed only at the same Paclitaxel concentrations required for mitotic arrest, suggesting that the mechanism underlying the synergic interaction was a Paclitaxel-induced alteration of cell cycle kinetics. This study stresses the importance of the administration sequence in the protocols involving NMF as a cytotoxic effect modulator as well as the role of cell kinetics in determining the effectiveness of this modulation.