Bacteroides fragilis grown in the presence of sub-inhibitory concentrations of clindamycin was shown to be altered its degree of encapsulation and susceptibility to phagocytosis by human polymorphonuclear leukocytes. Little polysaccharide capsule could be demonstrated either by light or transmission electron microscopy when the bacteria were grown anaerobically for four hours in the presence of 1/2 MIC of clindamycin. Such clindamycin-grown cells could be opsonized by normal human serum, and although less complement was consumed in the process, were more effectively taken up by the leukocytes than bacteria grown in the absence of the drug (45% versus 24%). It was also shown that drug treatment caused significant cellular leakage in the presence of serum, the 3H-label appearing extracellularly. In addition there was greater loss of viability of the bacterial cells grown in the presence of the drug and subsequently exposed to the leukocytes for 60 min.