Factors influencing MCMV infection mediated by MHC class 1 molecules were analysed further as previous studies showed that the effects of the MHC genotype on sensitivity to this virus are important in vivo. Here we show that H-2d, H-2b, H-2r and H-2v macrophages are highly sensitive to MCMV. Moreover, transfection of H-2k L-cells with Kb or Dd conferred sensitivity to MCMV. This was not affected by amino acid substitutions in Kb alpha 1 or alpha 2, although previous studies demonstrated that exchange of the alpha 1 domain of Dd with Ld alpha 1 compromised sensitivity. Here replacement of Kb alpha 3 with Ld alpha 3 reduced susceptibility to low doses of MCMV. In addition, extracellular beta 2-microglobulin (beta 2m) promoted infection of beta 2m-negative RIE/TL8X.1 cells transfected with Db with or without a beta 2m gene. Hence MCMV infection can involve beta 2m and the alpha 1 and alpha 3 domains of MHC heavy chains. MCMV infection of L-cells expressing Dd or Kb was also inhibited by heparin, but infection of the parental L-cell line was not reproducibly affected. A role for heparan sulphate proteoglycan in MHC-mediated MCMV infection was confirmed using cells pre-treated with heparinase I or III, or propagated in chlorate to inhibit the sulphation of the glycosaminoglycan chains.