The expression of H-2 and Moloney leukemia virus (MLV)-determined surface antigens of MLV-induced mouse lymphoma cells (YCAB) was studied during growth in vitro by indirect membrane immunofluorescence and complement-dependent, antibody-mediated cytotoxic sensitivity. In a growing cell population, the degree of antigenic expression was inversely related to the growth rate and cell volume. These findings suggest that the antigenic properties of YCAB cells are maximally expressed during the early interphase, presumably a part of the G1 period, and that the fast-growing cells pass relatively quickly through G1. The life cycle analysis of the same cells at varying intervals during the growth cycle revealed that the prolongation of population doubling time was mainly due to an extension of the G1 period, whereas the duration of S, G2, and mitosis was much less affected. The cell volume per se did not appreciably influence the expression of surface antigens. Populations of large cells were more rapidly growing, since they were in the late stage of their cycle.