The immune response to a primary infection of Heligmosomoides polygyrus (Nematospiroides dubius) was studied by flow cytometry in three strains of mice, BALB/c, CBA, and NIH. The chief feature of the response was a pronounced increase, during the first week, in the proportion of B lymphocytes in the mesenteric lymph nodes. All three strains also showed an increase in lymph node cellularity, although this was delayed in NIH and CBA mice. Total B cell numbers thus also increased, particularly in the BALB/c and NIH strains but only in the latter was this response maintained throughout the 4-week study. Although the early changes in B cell frequency were similar in all three strains, B cell responses were greatest in BALB/c mice, and most prolonged in NIH, when they persisted into the adult phase of the infection. These features distinguished them from CBA mice, and could be associated with known variations in resistance to challenge infections. An increase in T cell numbers was delayed in comparison with the changes in the B cell population, and the ratio of 'helper' to 'suppressor/cytotoxic' T cells remained more or less constant in all three strains. There was thus no evidence for an increase in the frequency of suppressor T cells in any strain of mouse.