Spleen cells from MRL-lpr and B6-lpr mice have a marked defect in the ability to produce interleukin 2 (IL-2) in response to concanavalin A stimulation. This defect precedes the onset of clinical illness, increases with age, and eventually becomes virtually absolute. It is not due to cellular suppression of IL-2 production, nor does it reflect the presence of a soluble inhibitor of IL-2 activity. Failure to restore IL-2 production with macrophage-replacing factors, such as interleukin 1 and phorbol myristic acetate, suggests that IL-2 deficiency reflects a primary T cell defect rather than a macrophage defect. MRL-lpr and B6-lpr spleen cells also have an age-dependent reduction in IL-2 response that apparently results from a deficiency of cell surface receptors for IL-2. Congenic MRL-+/+ and B6-+/+ mice, which lack the lpr gene responsible for accelerated autoimmunity and lymphoproliferation, have normal IL-2 activity. These findings suggest that a defect in IL-2 activity may contribute to impaired immunoregulation in mice bearing the lpr gene. The absence of such a defect in MRL-+/+ and B6-+/+ mice further suggests that a single autosomal recessive gene is responsible for IL-2 deficiency.