Intrapulmonary clearance of type III group B streptococcus (GBS) and related phagocytic recruitment was studied in preterm and term rabbits at 4 and 8 h after aerosol infection using left lung homogenates to quantify GBS and right lung bronchoalveolar lavage (BAL) to recover phagocytes. Opsonophagocytosis of type III GBS by pulmonary alveolar macrophages (PAM) was studied in vitro with a modified differential fluorescence-quenching method using PAM lavaged from preterm, term, and adult rabbits. Sera of experimental animals were tested for opsonization of type III GBS using fluorescein-labeled antibodies to rabbit IgG and C3. Although term animals showed some clearance at 8 h, preterm animals had marked intrapulmonary proliferation of GBS. The number of PAM in BAL fluid was 20-fold higher in term than in preterm rabbits at 0 h, but by 8 h, preterm rabbits had a large influx of PAM, whereas PAM remained constant in term BAL fluid. Rates of phagocytosis of GBS were twice as high in term versus preterm PAM during in vivo and in vitro studies. Among PAM from term and adult animals, opsonization of GBS with MgEGTA-sera promoted phagocytosis in vitro at levels comparable to pooled adult sera, whereas opsonization with EDTA-sera resulted in a significantly lower rate of ingestion. None of the experimental animals' sera were able to deposit IgG, but all deposited C3 on the surface of GBS. In summary, preterm rabbits had a diminished bronchoalveolar PAM population that did not ingest GBS as well via the alternative complement pathway when compared with term rabbits.(ABSTRACT TRUNCATED AT 250 WORDS)