Recombinant human interleukin-2 (rH IL-2, Ser^(125)-rH IL-2) purified from E. coli was characterized its biological and therapeutic activities for the establishment of preclinical screening system and therapeutic applications. In vitro augmentation of natural killer (NK) cell activity and mixed lymphocyte reaction (MLR) assay were shown to be comparable with other rH IL-2 from different manufacturers and laboratories. An antiserum against rH IL-2 was obtained from chick, and characterized the reacticvity with rH IL-2 reconstituted from lyophilized state in relation to the stability and conformational changes of the recombinant protein. In addition, long term storage of rH IL-2 in lyophilized or solution was not shown to incur remarkable loss of MLR activities, and any appearance of either degradation or aggregation products when analyzed by SDS-PAGE, indicating the stability and suitability for therapeutic applications of rH IL-2.