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Objective: Considering the differing mechaniisms of cytotoxicity produced by adriamycin and the photosensitizer meso-chlorin e6monoethylene diamine (Mce6) with light, the interaction of these agants in combination on human ovarian epithelial carcinoma (OVCAR-3 in vitro) was evaluated by dose and effect addition isobole analysis. Methods: Mitochondrial respiration via the 3-(4,5-dimethyl thiazeol-2 yl)-2,5-diphenyl tetrazolium bromide cleavage assay (MTT) and reproductive capacity via the tritiated thymidine incorporation assay (TI) were assessed 72 and 144 hours after exposure to adriamycin, Mce6, and light (650 nm), and to their combinations, in OVCAR-3 cells grown in vitro (20,000 cells per well). Results: In the majority of assays, reproductive capacity was more sensitive to the drug(s) than was mitochondrial respiration (2–10X). Dose-addition isobole analysis showed synergy for the combination of 50% median effective dose (ED50) adriamycin with 50% ED50Mce6/light in all assays (all P =.027). Antagonism was noted with the combination 25% ED50adriamycin with 75% ED50Mce6/light. Additivity and synergy were the predominant interactions for 75% ED50adriamycin with 25% ED50Mce6/light by dose-addition isobole analyses. Effect-addition isoboles showed a preominance of synergy, particularly for the combination 50% ED50adriamycin with 50% ED50Mce6/light. Conclusion: Synergy and additivity are the primary in vitro interactions for the combination of adriamycin and Mce6/light in the dosage range tested. Reproductive capacity is more sensitive to the these agents than in mitochondrial respiration.