Intranuclear B-capsids from cytomegalovirus (strain Colburn)-infected cells contain an abundant 37-kDa assembly protein, thought to be involved in capsid formation, and three minor protein constituents (i.e., 45, 39, and 38 kDa) that are immunologically and structurally related to the assembly protein. In the experiments reported here, antisera produced against synthetic peptides were used in conjunction with chemical protein cleavage to examine the structural relationship of these proteins in more detail. Results of these experiments verify that the carboxyl end of the 39-kDa assembly protein precursor is lost during maturation and suggest that the 38-kDa protein may be a processing intermediate. It is shown that the 45-kDa protein is coterminal with the mature assembly protein at its carboxyl end but differs by a predicted 115-amino-acid extension at its amino terminus. In addition, evidence is presented that the 45-kDa protein has a 48-kDa precursor and a 47-kDa putative processing intermediate which have the same carboxy-terminal sequences and undergo the same maturational events as those of the assembly protein. A working model considering the structural relationship of these proteins is presented.