The studies were conducted on non-infected and E. coli-infected mice treated with marbofloxacin (Marbocyl 10 % inj.) administered intramusculary five times at 24-h intervals at a dose of 2 mg/kg. It has been found that marbofloxacin augments the percentage of phagocyting cells in non-infected mice, but does not change the number of phagocytized bacteria. However, marbofloxacin impaires the killing activity of peritoneal macrophages in non-infected mice (decreased nitric oxide production) and E. coli-infected mice (decreased percentage of NBT-positive cells). Moreover, marbofloxacin enhances the synthesis and release of interleukin-1 (IL-1), both in non-infected and E. coli-infected mice. The administration of marbofloxacin has also been found to increase the percentage of immature CD4⁺CD8⁺ thymic cells and to decrease the percentage of double-negative (CD4^-CD8^-) and single-positive (CD4⁺ and CD8⁺) thymocytes. Administration of marbofloxacin does not change the percentage of T and B splenocytes and mesenteric lymph node cells in non-infected and E. coli-infected mice.