We have investigated the pharmacokinetic alteration in rats of recombinant interleukin-2 (rIL-2) by immunocomplexing with a monoclonal antibody against rIL-2. Serum rIL-2 levels after the intravenous administration of the immune complex at a dose of 100 micrograms/rat as rIL-2 were significantly higher than those after intravenous administration of rIL-2 alone at the same dose. Pharmacokinetic analysis indicated that the distribution volume of rIL-2 decreased from 74.0 to 10.3 ml/rat, while the elimination rate of rIL-2 was little changed by immunocomplexing with the antibody. On the other hand, serum rIL-2 levels after the subcutaneous administration of the immune complex at a dose of 100 micrograms/rat as rIL-2 were sustained longer than those after the subcutaneous administration of rIL-2 alone at the same dose, and Tmax shifted from 0.83 to 3.0 h by immunocomplexing with the antibody. Pharmacokinetic analysis also revealed that the mean-residence-time of rIL-2 increased from 1.98 to 6.52 h, and the area-under-the curve of rIL-2 decreased slightly, from 834 to 548 ng.h/ml, by immunocomplexing with the antibody.