Thirty-one type II and eight nontypable group B streptococcal isolates were categorized by the presence of the trypsin-resistant and trypsin-sensitive components of the Ibc protein antigen and studied for opsonization differences by using polymorphonuclear leukocyte chemiluminescence (CL). Type II strains varied in their ease of opsonization: strains possessing both components of the Ibc protein elicited a significantly lower mean CL peak (P ≤ .05) and a smaller CL integral (P ≤ .01) than did type II strains lacking both components or strains lacking type polysaccharide antigen but possessing the Ibc protein antigen. These strain differences were seen consistently with different sera and were also observed in an opsonophagocytic killing assay. Differences in opsonizing ability and bacterial killing were found for three standard test sera but could not be related to the concentration of type-specific antibody to capsular polysaccharide antigen. Our studies suggested that the Ibc protein antigen was one factor that contributed to the resistance to opsonization of strains of type II group B streptococci.