Intensive apoptotic death of thymocytes is a possible mechanism of thymus involution during tumor growth. We studied the role of hypercholesterolemia and lactate acidosis in the induction of increased sensitivity of thymocytes to apoptosis during growth of transplanted hepatoma 22a in mice. Spontaneous apoptosis in thymocytes during tumor growth in mice was studied in vitro by acridine orange/ethidium bromide staining and diphenylamine test. Plasma levels of lactate, total cholesterol, alpha-cholesterol, and triglycerides were measured. A positive correlation was found between intensification of apoptosis (diphenylamine test) and increased concentration of total plasma cholesterol on days 21 and 28 after inoculation of tumor cells. Plasma lactate content did not increase at this term. We hypothesize that hypercholesterolemia accompanying tumor growth acts as a factor increasing thymocyte sensitivity to apoptosis.