Localization of protein B23 in HeLa cells under different growth conditions was studied using indirect immunofluorescence. Bright nucleolar fluorescence was observed in exponentially growing cells. After 3 to 4 days, the initial cell inocula entered a phase of stationary growth as defined by no increments in cell number. The nucleolar fluorescence was then diminished and a general nuclear immunofluorescence was observed. This change in localization of fluorescence indicated that protein B23 had migrated out of the nucleoli during the suboptimal growth conditions. Relocation of protein B23 in nucleoli was observed in cells of stationary growth after treatment with adriamycin or daunomycin at their subtoxic concentrations (10(-10) M). Adriamycin and daunomycin, at their toxic concentrations (greater than 5.0 x 10(-7) M), on the other hand, inhibited cell growth and induced B23 translocation from nucleoli to nucleoplasm in growing cells. These results indicate that both adriamycin and daunomycin exhibit biphasic effects on the proliferation of cells by either stimulation or inhibition depending on the drug concentrations and the growth conditions. B23 translocation, as observed by immunofluorescence may be a simple and rapid method for assessing inhibition-stimulation growth response to anti-tumor therapy.