Capsids of human and simian strains of cytomegalovirus (HCMV and SCMV, respectively) have identified counterparts for all but one of the protein components of herpes simplex virus (HSV) capsids. The open reading frames (ORFs) for the CMV and HSV counterpart proteins are positionally homologous in the two genomes. The HSV capsid protein without a recognized counterpart in CMV is VP19c, a 50-kDa element of the intercapsomeric "triplex." VP19c is encoded by HSV ORF UL38, whose positional homolog in the HCMV genome is UL46. The predicted protein product of HCMV UL4A6, however, has essentially no amino acid sequence similarity to HSV VP19c, is only two-thirds as long, and was not recognized as a component of CMV capsids. To identify and learn more about the protein encoded by HCMV UL46, we have expressed it in insect cells from a recombinant baculovirus and tested for its presence in CMV-infected human cells and virus particles with two UL4A6-specific antipeptide antisera. Results presented here show that this HCMV protein (i) has a size of approximately 30 kDa as expressed in both recombinant baculovirus-infected insect cells and HCMV-infected human cells; (ii) has a homolog in SCMV; (iii) is a capsid component and is present in a 1:2 molar ratio with the minor capsid protein (mCP), encoded by UL85; and (iv) interacts with the mCP, which is also shown to interact with itself as demonstrated by the GAL4 two-hybrid system; and (v) aggregates when heated and does not enter the resolving gel during sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE), a characteristic that accounts for it eluding detection until now. We call this protein the mCP-binding protein, and on the basis of the characteristics that it shares with HSV VP19c, we conclude that the HCMV mCP-binding protein is the functional as well as genetic homolog of HSV VP19c.