Unlike other small animals, cytomegalovirus (CMV) infection of the guinea pig results in transplacental passage and intrauterine infection of the fetus. These features make the guinea pig model ideal for studying vaccine strategies designed to prevent congenital infection. Unfortunately, little is known about immunogenic guinea pig CMV gene products. In other animal cytomegaloviruses, a major target of the host immune response is the glycoprotein B (gB, gp UL 55) gene product. Using DNA probes containing human CMV gB sequences, the gB gene homolog of the guinea pig cytomegalovirus was identified, cloned, and sequenced. The gpCMV gB gene maps to a region spanning portions of the HindIII K, Q, and P fragments of the gpCMV genome. DNA sequence analysis identified an open reading frame of 2706 nucleotides capable of encoding a protein of 901 amino acids. Extensive similarity to the human and murine gB proteins was noted with 42% identity at the amino acid level. The predominant gpCMV gB mRNA is a 6.8-kb transcript with the expression kinetics of an early gene. RNase protection and primer extension analyses indicated that gB mRNAs were transcribed from two different initiation sites corresponding to distinct TATA elements. Polyclonal antisera prepared against a synthetic peptide derived from amino acid sequences within the ORF identified a 58-kDa virion-associated protein representing the cleaved COOH-terminus (gp 58) of the gpCMV gB molecule. The molecular characterization of gpCMV gB should facilitate studies of vaccine strategies in the guinea pig model of congenital CMV infection.