In response to primary and secondary stimulation by xenogeneic HeLa carcinoma cells or by syngeneic sarcoma MSVC cells the regional lymph nodes of BALB/c mice are enlarged, however, their absolute mast cells content does not increase and in relative terms a decrease of mast cells concentration is observed. These results are in accordance with our earlier observations that during antigeneic and T-cell mitogen stimulation the lymphatic mast cells population does not increase but is rather reduced. This may indicate the engagement of these cells in delayed-type hypersensitivity.