The effect of vitamin A deficiency on the response of splenic lymphocytes to mitogenic stimulation was determined in an experimental rat model. Male Lewis rats were divided into three groups. The ad libitum group (AL) was fed unlimited amounts of a vitamin A-supplemented diet. The vitamin A-deficient group (DEF) received a commercial vitamin A-free diet. The pair-fed group (PF) received a vitamin A-containing diet equivalent in amount to that consumed by the DEP group. During the early stages of vitamin A deficiency (determined by cessation of weight gain), the rats were killed and the isolated splenic lymphocytes subjected to mitogenic stimulation. Lymphocytes from DEF rats had one-third the transformation response to the mitogens Concanavalin A, Phytohemagglutinin and E. coli Lipopolysaccharide S of the AL and PF groups. When the DEF rats were supplemented with vitamin A, the transformation response returned to control values within 3 days. In addition to the alterations in the immune response, the DEF rats showed a marked leukopenia, a decrease in the number of circulating lymphocytes and an increase in the number of circulating neutrophils.