Inbred strains BALB/c and CBA and noninbred Swiss mice were used for transplanting skin autografts exposed before transplantation for approximately 48 h to (a) culturing in room air and (b) culturing in 45–48 % CO2 in air. These pre-transplantation manipulations of skin autografts led to malignant lymphomas in the hosts. The lymphoma incidence was highest in the recipients of the CO2-treated grafts; moreover, it was greater than the spontaneous incidence even in the recipients of grafts exposed to culturing in air alone, in some types of mice. The following abnormalities of the reticuloendothelial system were noted: (1) proliferation of lymph follicles into irregular masses of pleomorphic cells, (2) hyperplasia with concomitant atrophy of the lymphoid tissue, (3) replacement of the lymph follicles by malignant lymphoid cells. Renal lesions resembling membranous glomer-ulitis occurred occasionally. The occurrence varied in different strains.