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A radio-immunoassay for human T cell growth factor, also called Interleukin-2 (IL-2), has been carried out using a recombinant IL-2 preparation as tracer and a polyclonal rabbit antiserum. The assay is highly specific for IL-2: there is no cross-reaction with either type I and II interferons, epidermal growth factor or tumor necrosis factor alpha. Using the sequential saturation procedure the limit of sensitivity was 0.5 U/ml. Intra- and between-assay coefficients of variation were 8 and 11%, respectively. With this assay, IL-2 recovery in serum and peripheral blood mononuclear cell culture (P.B.M.C.) medium was 79 and 95%, respectively. In serum of 109 normal subjects and 102 rheumatoid arthritis patients mean IL-2 concentrations (+/- SD) were 1.5 +/- 0.5 U/ml and 1.4 +/- 0.4 U/ml respectively. The IL-2 production by P.B.M.C. in vitro was also studied. In unstimulated cultures, IL-2 release remained undetectable, i.e. below 0.5 U/ml. After stimulation of mononuclear cells from 36 normal subjects with increasing amounts of phytohemagglutinin (PHA), the 3H-thymidine incorporation followed a bell-shaped curve, the maximum response being observed at a 2.5 micrograms/ml PHA concentration. After a 72-hr mononuclear cell stimulation, IL-2 release increases with PHA concentrations ranging from 0 to 10 micrograms/ml. In patients with rheumatoid arthritis (R.A.), P.B.M.C. incorporated 3H-thymidine as in normal subjects. In contrast, mean +/- SEM IL-2 production by P.B.M.C. from patients with inactive RA (5 +/- 0.9) and active disease (1 +/- 0.5) was significantly lower than that from normal subjects (12 +/- 0.7 ng/ml).(ABSTRACT TRUNCATED AT 250 WORDS)