The cytokine levels of soluble interleukin-2 receptor (sIL-2R), interleukin-6 (IL-6) and tumor-necrosis-factor alpha (TNF-alpha) were studied in 12 healthy volunteers at 11 different times of day. TNF-alpha levels were below the detection limit, and IL-6 levels were at baseline values of the respective assay used. Interindividual variations were found for the plasma levels of sIL-2R (179-524 U/ml). Shedded IL-2 receptors displayed a pronounced circadian phase-dependency (p less than 0.0001) with a peak value at 12:29 h and a trough at 4:14 h when a complex cosine function (period lengths: 24 h plus 12 h) was fitted to the data. These findings may be of importance when using sIL-2R as a diagnostic tool as well as in controlling efficacy of drug treatment.