In defining host resistance factors in uremia, experiments were designed to assess the effect of renal failure serum upon the reactivity of normal human lymphocytes to phytohemagglutinin in vitro. Normal buffy coat cells were resuspended in sera obtained from normal subjects and from 14 patients with renal failure, then stimulated with phytohemagglutinin M and the cellular response measured by the increase in thymidine or uridine uptake. The mean thymidine uptake by stimulated cells in normal sera was 14,389 +/-1695 (SEM) cpm per 2 x 10(6) lymphocytes. Uridine uptake under the same conditions was 12,540 +/-1887 cpm. Compared to these are a mean thymidine uptake of 2740 +/-457 cpm and uridine uptake of 3928 +/-667 cpm in renal failure sera. Both differences are significant at P<0.01 level. For controls representing "chronic illnesses," sera from patients with pneumococcal meningitis, cirrhosis of the liver without jaundice, rheumatoid arthritis, and paraplegia with urinary tract infection did not cause suppression. No single drug had been taken by all the renal failure patients; three patients were taking no drugs. The serum from one patient with acute renal failure suppressed thymidine uptake while her serum obtained after recovery from her illness supported a normal lymphocyte response. Improvement of lymphocyte response was also noted in 9 of 10 sera obtained from patients immediately after hemodialysis. These observations plus the inhibition of stimulated cells by normal serum mixed with renal failure serum indicate the presence of a dialyzable inhibitory factor rather than the absence of a supporting factor in the renal failure sera. Lymphocytes preincubated for 24 hr in renal failure serum responded normally when transferred to normal serum and stimulated. Cells stimulated in normal serum and transferred to renal failure serum within the initial 24 hr of incubation demonstrated depressed thymidine uptake. Also, cell survival for 72 hr incubation as judged by trypan blue exclusion and chromium-51 release was similar in normal and renal failure sera. Thus, the suppressive effect of renal failure serum does not depend upon the initial phytohemagglutinin-cell interaction nor upon a significant cytotoxic effect. These studies demonstrate that a dialyzable factor(s) in the serum of patients with renal failure can greatly suppress one parameter by which an immune function of circulating lymphocytes is assessed and provides at least, a partial explanation for delayed homograft rejections in renal failure as well as the susceptibility of such patients to various infections.