Culturing Escherichia coli, Proteus mirabilis and Klebsiella pneumoniae in the presence of subinhibitory concentrations (1/4 MIC) of trimethoprim, sulphamethoxazole or their combination, resulted in reduced adherence of all the above strains. The number of phagocytosed bacteria pre-exposed to subinhibitory concentrations of the above antibiotics was not significantly changed, but a significant increase of bactericidal activity of the polymorphonuclear leucocytes was observed. Furthermore, filtrates of K. pneumoniae and P. mirabilis grown in the presence of trimethoprim alone or in combination with sulphamethoxazole induced an increased chemotactic response of human polymorphonuclear leucocytes.