SUMMARY Mouse lymphoma cells of different lines were incubated in vitro at 37 C for periods varying between 5 and 240 min. At regular intervals, aliquots were removed, washed, and tested for sensitivity to the cytotoxic action of anti-H-2 sera, in the presence of complement. Cytotoxic sensitivity showed a cyclic variation during incubation, with repeated high and low values. The ability of the cells to absorb the corresponding H-2 antibodies varied inversely with cytotoxic sensitivity. It was concluded that the expression of H-2 isoantigens fluctuates in a periodic fashion during the conditions of incubation.