The effects of sequential combinations of N-methylformamide (NMF) with adriamycin (ADM) on a human melanoma cell line (M14) were investigated. The results obtained can be summarized as follows: (i) When NMF was administered before ADM (NMF-->ADM), a decrease of ADM-induced cytotoxicity and intracellular ADM content was revealed. Conversely, the reverse combination (ADM-->NMF) produced a remarkable reduction of cell survival accompanied by a significant intracellular drug retention. (ii) The fluorescence microscopy revealed that in NMF- and in NMF-->ADM-treated cells the actin microfilament distribution appeared to be very similar to that in control cells. On the contrary, a disorganization of the microfilament architecture was observed in ADM-->NMF-treated cells. (iii) The microtubule organization was significantly altered by NMF. This effect appeared to be even more evident in M14 cells treated with the combination ADM-->NMF. (iv) Confocal microscopy observations showed an intracytoplasmic retention of ADM in ADM-->NMF-treated cells. These findings suggest that the cytoskeletal changes induced by NMF might interfere with the ADM efflux mechanisms, accounting for the high drug level and low survival detected in ADM-->NMF-treated cells.