AbstractThe effects of N‐(4‐hydroxyphenyl)retinamide (HPR), a synthetic analogue of vitamin A, on cell morphology, cell cycle kinetics, cytoplasmic matrix, and expression of murine mammary tumor virus (MuMTV) in MuMTV‐infected murine mammary tumor cells (GR‐3A) were determined. Cellular uptake of HPR was rapid and linear, with zero‐order kinetics, during the first 30 minutes of incubation. Flow cytometric analysis of cells treated with nontoxic levels of HPR (10μM)for 48 hours revealed a reduction in percent cells in the DNA synthetic (S) phase of the cell cycle with a concomitant increase in percent cells in the G, phase of the cell cycle. Dexamethasone‐stimulated MuMTV expression was not affected after 48 hours of HPR exposure, whereas the virus expression was significantly reduced in cells treated with HPR for seven days. The reduction in MuMTV expression was preceded by changes in cell morphology (decreased cell‐cell contact and reduced cell flattening) and altered F‐actin aggregation. Continuous exposure to HPR (10μM) for 14 days resulted in reduced cell proliferation rates and decreased cell plating efficiency of GR‐3A cells. Taken together, these results indicate that HPR is rapidly incorporated into GR‐3A cells and that the effects of HPR on celt profileration, cytoskeletal organization, and cell morphology appear to precede the effects of this retinoid on the expression of the etiological agent of murine mammary tumorigenesis, MuMTV.