Mouse red blood cells (RBC) were incubated with recombinant human interleukin 2 (rIL 2) in protein-free media. Of the added cytokine, about 20% was bound to the cells. Dissociation of rIL 2 from RBC commenced when the carrier cells were transferred into fresh media. The release from the RBC vehicle was enhanced in the presence of soluble protein. When rIL 2-coated RBC (RBC-IL2) were incubated in protein-containing media for 24 h to release the majority of bound rIL 2, and then were subsequently incubated in fresh media with an IL 2-dependent cytotoxic T cell line (CTLL), residual bound rIL 2 was still released as demonstrated by CTLL proliferation. RBC were covalently coupled to specific monoclonal antibody (mAb) towards the lymphocyte cell-surface marker Thyl.2, coated with rIL 2, subsequently exposed to the target CTLL cells, then evaluated for CTLL/RBC rosette formation. The attachment of mAb to the RBCs surface did not markedly change the succeeding rIL 2 adsorption, and the bound rIL 2 did not impair antigen recognition by the mAb. At higher rIL 2 concentrations (1200-1500 i.u./10(8) cells), RBC-IL2 plus Thy1.2 mAb provided higher CTLL response than when RBC-IL2 plus non-specific mAb was used thus demonstrating enhancement by targeting. However, the targeting effects were not seen with lower rIL 2 concentrations (80-110 i.u./10(8) cells).