Thymidine incorporation by human blood mononuclear cells, unstimulated and stimulated with phytohaemagglutinin (PHA), pokeweed mitogen (PWM) and purified protein derivate of tuberculin (PPD) was studied with and without addition of pyrimethamine. Responses were increased significantly by pyrimethamine in the PWM and PPD stimulated cultures, in the PPD cultures even by concentrations of pyrimethamine, which can be achieved with therapeutic doses of this drug. The closely related compound trimethoprim did not alter PHA‐, PWM‐ and PPD‐responses significantly. The possibility of using pyrimethamine ‐ a widely used prophylactic antimalarial ‐ as an immunopotentiator is mentioned. 1981 Acta Pathologica, Microbiologica et Immunologica Scandinavica