Summary Localization of actin was studied in HL-60 leukemia cells after treatment with the anticancer agent etoposide for 3 days in a range of concentrations (0.02- 200 μM). Significant changes in morphology of the cells and F-actin distribution patterns labelled with TRITC-phalloidin occurred only after treatment with 100 and 200 μM etoposide. In comparison with control cells, the number of cells decreased, cells were larger and almost all treated cells had irregular surfaces with lamellipodia. F-actin was distributed in a punctate pattern throughout the cytoplasm after treatment. In some treated cells, fluorescence appeared as a bright haze, whereas in other cells it formed a network. Treated cells also showed bright fluorescence at their periphery. Immunogold labelling of actin was observed in cells whether or not treated with etoposide. Labelling was found in the nucleus and also in the cytoplasm. At the ultrastructural level, cells treated with 100 and 200 μM etoposide showed increased positivity for actin in relation with blebbing, margination of nuclear chromatin and bodies containing recognizable nuclear fragments. These findings indicate that alterations in expression of actin in HL-60 cells after treatment with etoposide is dose-dependent and related with apoptosis.