Introduction and objective: In in vitro cell line studies, two isobolographic analysis approaches are commonly used when assessing interactions between drugs. Both – the log-probit accompanied with Tallarida method and Chou-Talalay-Martin method – can distinguish pharmacodynamic types of interactions occurring between drugs. Material and methods: The aim of the study is to assess the interaction between LY-2183240 and mitoxantrone in the malignant melanoma SK-MEL28 cell line in the MTT assay, by means of two methods – isobolographic (log-probit) accompanied by the Tallarida and Chou-Talalay-Martin method. Results: Log-probit accompanied by the Tallarida method revealed that LY-2183240 and mitoxantrone had non-parallel concentration-response curves, and produced an additive interaction in the SK-MEL28 cell line in the MTT assay. In contrast, the Chou-Talalay-Martin method showed that the combination of LY-2183240 and mitoxantrone exerted a synergistic interaction with a combination index value of 0.354. Conclusions: The observed discrepancy in the types of interactions between LY-2183240 and mitoxantrone in both methods resulted from the lack of assessment of mutual parallelism of the drugs concentrations-response curves in the Chou-Talalay-Martin method. The utmost caution is advised when utilizing the Chou-Talalay-Martin method in the detection of interactions in in vitro studies, especially for the combinations of drugs with non-parallel concentration-response curves.