The effect of pre-incubation of Pseudomonas aeruginosa, Klebsiella pneumoniae and Proteus mirabilis with sub-inhibitory concentrations (sub MICs) of clindamycin on the adherence of these organisms was studied. Culturing these organisms in the presence of clindamycin (4 mg/l) resulted in significant enhancement of adherence for Ps. aeruginosa and Pr. mirabilis and decreased adherence for K. pneumoniae. Furthermore, the effect of pre-exposure to clindamycin on polymorphonuclear leukocyte (PMNLs) function against these organisms was determined. Filtrates of Pr. mirabilis pre-exposed to clindamycin promoted PMNL chemotaxis. No effect on chemotaxis was noted with the filtrates of clindamycin treated Ps. aeruginosa and K. pneumoniae. PMNL phagocytosis for all the organisms was increased after they were pre-exposed to clindamycin.