Soluble interleukin-2 receptor (sIL-2R) concentrations were measured in the sera of 18 healthy adult volunteers, 40 healthy children aged 5 months -16 years 8 months and cord blood from 10 babies, using two commercially available ELISA kits (T-cell Sciences and Serotec). In the presence of normal C-reactive protein (CRP) children (aged 0-16 years 8 months) had significantly higher sIL-2R concentrations than adults aged (18-37 years). In children up to the age of 16 years there was a significant negative correlation between age and sIL-2R (r = -0.725 p < 0.0001 T-Cell Sciences, r = -0.646 p < 0.0001 Serotec). A comparison was also made between the two kits, which indicated that both kits yielded reproducible results but the T-Cell Sciences kit had a larger working concentration range than the Serotec kit. Following 12-h sampling in adult volunteers, a lower concentration of sIL-2R was observed at 4 am compared with 10 am (p = 0.0052) which indicates diurnal variation. These observations emphasize the importance of the study of normals as part of any research, and raises further questions regarding the many factors which influence immune function.