Following consumption of 1.2 litres of cows’ milk by normal human adults there was a rapid fall in the proportion of peripheral blood lymphocytes bearing receptors for the reacted Fc of IgG (Fcγ-receptors). This phenomenon was transient and apparently confined to the lymphocyte Fcγ-receptor+ve subpopulation. Similar observations were made in rats but only in those animals pre-exposed to cows’ milk suggesting that an immunological mechanism is involved. In man it was found that Fcγ-receptors could only be re-expressed following incubation of post-milk lymphocytes in normal human serum. It is proposed that rapid in vivo modulation of lymphocyte Fcγ-receptors occurs following oral antigen (cows’ milk) challenge probably mediated by soluble food antigen-antibody complexes. The subsequent recovery of these receptors in vivo and in vitro may be due to the binding of ‘fluid-phase’ Fcγ-receptors found in normal human serum.