We studied the production of the two major mediators of cellular immune responses, Interleukin 1 (IL-1) and Interleukin 2 (IL-2), by the peripheral blood mononuclear cells of 23 burn patients (16 men, seven women, mean age 48.9 years) compared with 23 matched controls (16 men, seven women, mean age 46.7 years). Serial measurements were made of IL-1 production by adherent mononuclear cells after stimulation with lipopolysaccharide and of IL-2 production by lymphocytes after stimulation with phytohemagglutinin (PHA). Eighty determinations of IL-2 production by lymphocytes from 12 patients with greater than 30% body surface area burn revealed a mean IL-2 production of 0.71 u as compared with a mean of 1.23 u for patients with less than 30% burns (p = 0.04). Patients with greater than 30% body surface area burns had significantly reduced IL-2 production (p less than or equal to 0.05) until 60 days after injury, whereas those with smaller burns had reduced IL-2 production only at 20-29 and 30-39 days postburn. Nine burn patients with systemic sepsis showed significantly lower IL-2 production (p = 0.03) at 10-29 days postburn than nonseptic patients, and significantly less IL-2 production during septic episodes. Eight patients with greater than 50% suppression of lymphocyte response to PHA produced less IL-2 (0.4 u) than patients with less than 50% suppression, (1.07 u, p = 0.004). IL-1 production was significantly elevated as compared with controls (4.45 u vs. 3.6 u, p = 0.05) early after injury, but was subsequently within the normal range regardless of burn size. The percentage of circulating helper T-lymphocytes, the principal source of IL-2, was also reduced, although this did not always correlate with IL-2 production, which remained depressed after recovery of the helper T-cell population. These results indicate that failure to produce IL-2, a powerful mediator of cellular immune responses, is an important mechanism underlying the defective cell mediated immunity seen in burn patients.