The study of cytograms of the marrow of mice with experimental T. spiralis and T. pseudospiralis invasion of 1 to 60 days revealed the marked activation of granulocytes and lymphoid cell proliferation with the much lower response of erythroid cells. The maximal response occurred at 21-35 days of invasion with subsequent diminution. In mice experimentally invaded with T. pseudospiralis the response of the marrow was less pronounced, but the period of active cell proliferation was prolonged.