Peptidoglycan-polysaccharide polymers derived from group A streptococcal cell walls (PG-PS) were opsonized with either purified C3 or normal human serum and were used as a probe to investigate the mobility of CR1 and CR3, the C3b and iC3b receptors, respectively, on human neutrophils. Incubation of monolayers or cell suspensions of neutrophils with PG-PS opsonized with C3b or serum resulted in capping of PG-PS, as detected by fluorescein-labeled antibody to PS. No binding of PG-PS to neutrophils was observed with heat-inactivated serum. By 30 min the cell walls were internalized and observed in one to three vacuoles. Capping was totally inhibited when PG-PS opsonized with C3b or serum was preincubated with Fab'-anti-C3b. Similar inhibition was observed when C3b-opsonized PG-PS was incubated with neutrophils that were preincubated with anti-CR1 or fluid-phase C3b; only partial inhibition of neutrophil capping was observed by using serum-opsonized PG-PS. Because anti-CR1 blocks only the C3b receptor, the cap formation observed with serum-opsonized PG-PS is probably due to CR3. These results suggest that both CR1 and CR3 on neutrophils cap after stimulation by group A streptococcal cell wall fragments.