Imipenem, a new carbapenem, was tested for its inhibitory or stimulatory effects on concanavalin A- and phytohaemagglutinin-stimulated and unstimulated proliferation (measured by 3H-thymidine uptake) of murine splenocytes and thymocytes. Addition of imipenem at concentrations of 10-50 mg/l induced lymphocyte transformation in unstimulated and mitogen-stimulated lymphocytes. Since imipenem stimulated blastogenesis at concentrations achievable in serum when used therapeutically, these results may have potential clinical significance.