Quantitative study of doxorubicin (Adriamycin), pirarubicin (4'-o-tetrahypyranyladriamycin) and daunorubicin in the nucleus of living cells was performed using microspectrofluorometry. As for the cytotoxic assays, drug-sensitive and drug-resistant K562 cells were incubated for 3 days with concentrations of drug ranging from 4 nM to 1 mM. When drug-sensitive cells were incubated with pirarubicin, the spectrum recorded from inside the nucleus was characteristic of anthracycline intercalated between the base pairs in the nucleus. However, when drug-sensitive cells were incubated with doxorubicin and drug-resistant cells with pirarubicin or doxorubicin, a new fluorescent spectrum was obtained which was due to 7,8-dehydro-9,10-desacetyldoxorubicinone, a pirarubicin and doxorubicin degradation product that is formed in the medium. This compound which is highly lipophilic is taken up rapidly into both sensitive and resistant cells.