Human cytomegalovirus can support an abortive infection in Vero cells and only some immediate early events are expressed in a low proportion of the cell population. It has been shown that cellular factors partially remove blocks in cells abortively or latently infected by HCMV, whereas 5-azacytidine is known to act on gene expression and cell differentiation. In this work we present evidence that the treatment of HCMV infected Vero cells with 5-azacytidine induces the expression of the major immediate early protein (68K) of Human cytomegalovirus by enhancing a 72K cellular heat shock protein.