Mice chronically injected with amphetamine (0.4 mg/kg/day) showed a reduction in thymus and spleen cellularity, and in peripheral T lymphocyte population. The blastogenic response of spleen lymphoid cells was assessed and amphetamine was found to inhibit T-cell proliferation. Amphetamine also reduced the capacity of mice to the development and passive transfer of immunity to Listeria monocytogenes.