To study the roles of beta 2-microglobulin (beta 2-m) and major histocompatibility complex (MHC) class I expression in human cytomegalovirus (HCMV) infection, the ability of HCMV strain AD-169 to infect and replicate in a human melanoma cell line (FO-1), which is beta 2-m-deficient and cannot express MHC class I on its cell surface, was examined. Susceptibility of FO-1 cells was compared with human foreskin fibroblasts (HFF) and FO-1H cells (FO-1 cells that have been transfected with the human beta 2-m gene, restoring MHC I expression on the cell surface). As judged by the HCMV immediate early 1 (IE-1) antigen expression, HCMV was able to infect FO-1 cells, although somewhat less efficiently than HFF. However, the expression of HCMV late (L) antigen and the production of virus was significantly less for FO-1 cells than for HFF. Analysis of the FO-1H transfectants revealed that expression of IE-1 and L HCMV antigens was comparable to FO-1 cells, which lack MHC I. Treatment of FO-1 and FO-1H cells with sodium butyrate prior to inoculation did not alter the expression of MHC I in either cell type, but did increase susceptibility of both cell types to HCMV infection, as well as the expression of L antigens and production of virus. These studies indicate that HCMV infection of FO-1 cells is independent of beta 2-m and MHC class I expression.