The cellular uptake, subcellular distribution and retention of MX2, a new morpholino anthracycline, were compared with those of adriamycin (ADM) using confocal laser scanning microscopy (CLSM) in rat C6 and human T98G glioma cell lines. The tumor cells were exposed to 1-30 micrograms ml-1 of MX2 and ADM for 120 min and further incubated without drugs for 120 min after washing twice with medium. During incubation, real-time subcellular distribution of MX2 and ADM in living tumour cells were observed at various intervals using CLSM. For analysis of the in vivo uptake. Wistar rats bearing the C6 glioma were intravenously administered MX2 at a dose of 5 mg per kg body weight 60 min before sacrifice. The fluorescence of MX2 was predominantly seen in the cytoplasm in both C6 cells and T98G cells, although it was also present in the nucleus. In contrast, that of ADM was mainly confined to the nucleus in both cell lines. The fluorescent intensity of ADM in the nucleus after 120 min of exposure was approximately 1.5-fold higher than that of MX2 at the same dose exposure, probably indicating a greater amount of ADM accumulated in the nucleus than MX2. The influx and efflux of MX2 were much more rapid and greater than those of ADM in both cell lines. There was almost no difference in subcellular distribution among the doses tested in this study. The subcellular distribution of MX2 in vivo was almost similar to that of MX2 in vitro. These results suggest other mechanisms by which MX2 exerts its cytotoxic effects on tumour cells together with the inhibition of DNA topoisomerase II, which has been reported previously. It is considered that the CLSM technique is useful for the study of the cellular pharmacokinetics of antitumour agents such as anthracycline derivatives.