We have previously shown that an increased number of Escherichia coli/Bacteroides fragilis intraabdominal abscesses are produced in mice after preexposure to small numbers of live E. coli or B. fragilis. Splenic lymphocyte subset changes and the importance of different elements of the immune response in this system were studied. Preexposure to bacteria induced a significant increase in the percentage of splenic T cells without altering the CD4/CD8 ratio. The passive transfer of either 107 mixed splenic lymphocytes, 5 × 106 mixed T cells, or 2.5 × 106 CD4+ T cells from preexposed animals to naive siblings 24 hr prior to abscess induction resulted in increased abscess formation. Transfer of serum, B cells, <107 lymphocytes, CD8+ T cells, or any cell type from naive animals did not change abscess number. The bacterial composition of abscesses changed only in animals receiving either serum or B cells from donors preexposed to B. fragilis, where an increased number of B. fragilis per abscess was found. The CD4' T cell response can be altered by transient infections and is critical to subsequent abscess formation, and a concurrent humoral response may play a role in determining an abscess' ultimate bacterial composition.