5-Azacytidine was found to induce the expression of BALB:virus-1 and BALB:virus-2 from K-BALB cells and ecotropic endogenous virus from AKR2B cells. Efficiency of the induction was high and comparable to that by 5-bromodeoxyuridine. The level of methylcytosine in newly synthesized DNA was drastically decreased when K-BALB cells were treated with 5-azacytidine. There was an inverse relationship between the level of DNA modification and the frequency of virus expression.