The effect of sodium butyrate (NaB) alone and in combination with a glucocorticoid was studied on Y-79 retinoblastoma cells both in vitro and in vivo. Both NaB (0.5 and 4 mM) and hydrocortisone (1-100 microM) added separately to cells grown in suspension resulted in a small growth inhibition (less than 15%). A marked synergistic effect (75-77% growth inhibition) was observed in vitro when NaB and hydrocortisone were added in combination at all the concentrations tested. With in vivo experiments using the nude mouse model of retinoblastoma, the combination of NaB and methylprednisolone did not inhibit tumor growth.