Differences in the properties of homologous intracellular structural components of eight strains of subtype A and eight strains of subtype B of human respiratory syncytial (RS) virus were examined. The size of the fusion (F) protein cleavage products and the phosphoprotein (P) showed systematic differences between virus strains representing the two subtypes. The apparent mol. wt. in SDS-polyacrylamide gels under reducing conditions was 48,000 (48K) and 46K to 47K for the cleavage product F1 in subtype A and B strains, respectively. The size of the F2 protein was 18K to 20K. The subtype B strains showed a slightly higher mol. wt. of this protein compared to the subtype A strains. The size of the P protein was 36K in subtype A strains, but only 34K in subtype B strains. Variations also occurred in the size of the glycoprotein (G) and the 22K to 24K structural protein. These variations did not correlate with the virus subtypes, but were strain-specific. The size of non-glycosylated forms of the F protein cleavage products was determined by use of material from tunicamycin-treated cells. A 44K to 45K non-glycosylated form of the F1 protein was detected with subtype A virus strains, but the corresponding protein of subtype B strains was not reproducibly identified, presumably due to instability in the absence of glycosylation or altered antigenicity. Monoclonal antibody immunosorbent-bound viral glycoproteins were partially digested with proteases. The pattern of breakdown products of the F1 protein was distinctly different between subtype A and B strains, but it was similar among strains of the same subtype. No subtype-specific pattern was seen in proteolytic digests of monoclonal antibody-bound G protein.