Using bromodeoxyuridine (BrdU) and an anti-BrdU monoclonal antibody (Mab), the fraction of cells in S phase of the Y79 human retinoblastoma line maintained in an organ culture system was determined. The average labeling index (LI) (S phase cell fraction) was 21.5% after 2–15 days in vitro. The effect of butyric acid, which has been reported by others to inhibit the growth of Y79 retinoblastoma cells, was then evaluated in a similar in vitro system. Following treatment with 2 and 3 m M of butyric acid, extensive necrosis was seen in the cultures, and the LI of the surviving cells was significantly (p < 0.01) lower (4.1 and 2.9%, respectively) than that of the control cultures (21.1%). Forty-eight hours after removal of butyric acid, the LI of the cultures treated respectively with 2 and 3 m M of butyric acid returned to the control level. Our findings suggest that the effect of butyric acid on the cell kinetics of Y79 retinoblastoma cells may be a reversible blocking effect on the G<sub>1</sub> phase of the cell cycle.