The opsonization and phagocytosis of group B streptococci of types Ia, Ib, and Ic were studied in vitro by measuring the uptake of radioactivity by coverslip cultures of rabbit alevolar macrophages during incubation with radiolabeled, nonviable bacteria which had been exposed to rabbit serum. The uptake of counts per minute was quantitative, reproducible, and reversibly inhibited by cold, indicating that it was largely a measurement of phagocytic ingestion rather than of attachment of bacteria-immunoglobulin complexes to macrophage membranes. Moreover, suspended macrophages killed approximately 90% of viable streptococci in the presence of specific antiserum. The opsonic activity of immune serum was heat stable, and phagocytosis of streptococci was insignificant after incubation with normal serum and antiserum to some heterologous group B streptococci. By absorption studies, it was possible to identify the effect of antibodies to specific bacterial antigens. Phagocytosis of streptococci containing the corresponding antigens was maximal after opsonization with homologous or heterologous sera containing antibody to IaCHO, IbCHO, or Ibc protein. Phagocytosis of all three serotypes was intermediate when opsonization could be attributed to anti-IabcCHO. The opsonization of a specific group B streptococcus is complex and may involve two or more antigen-antibody systems.