Recombinant interleukin-2 (rIL-2) is able to maintain the growth and multiplication of T-lymphocytes and IL-2 dependent CTLL-2 cells up to 28 days with the multiplication of T-lymphocytes as high as about 1000 fold. When rIL-2 and its monoclonal antibody were added together, the above functions of rIL-2 were specially blocked, showing that these biological functions were exerted specifically by rIL-2. The rIL-2 is also able to increase the natural killer (NK) activity up to 46-96%, while higher concentrations of rIL-2 might exhibit inhibitory effect. The cytotoxicity of lymphokine activated killer (LAK) cells induced by rIL-2 for killing HL-60 cell line and solid tumour (human lung carcinoma) reached 54.84% and 37.96%, respectively. All these results indicate that the biological functions of rIL-2 were quite similar to that of the natural one.