The effects of free ampicillin, microencapsulated ampicillin anhydrate (MEAA) and antibiotic-free microspheres on the cell-mediated immune response in Balb/c mice were measured by lymphoproliferation assay, delayed-type hypersensitivity (DTH) and cytokine production. Injection into mice for seven consecutive days with equivalent subcutaneous doses of ampicillin, MEAA or placebo microspheres did not produce any consistent change in lymphocyte proliferation nor did it affect DTH responses or interleukin-2 production. Although the production of interleukin-4 in mice treated with ampicillin or MEAA increased compared with the control mice, this increase was not statistically significant. These results indicate that ampicillin and MEAA have similar effects on cell-mediated immunity in mice.