The mouse T-cell line CTLL-2 is known to be dependent on interleukin-2 (IL-2) for both growth and viability. These cells possess high affinity IL-2 receptors and have been shown to internalize IL-2 after binding. To determine if internalization of IL-2 is required for the mediation of its signal, IL-2 was covalently coupled to an insoluble matrix via glutaraldehyde cross-linking and CTLL-2 cells were incubated with the immobilized lymphokine matrix. This covalent cross-linking prevents the free lateral diffusion and internalization of the bound IL-2 receptors (IL-2R) while still permitting specific binding between the cells and the immobilized ligands. Although only very limited proliferation was observed during the incubation as assessed by 3H-thymidine incorporation, the viability of the CTLL-2 cells on the immobilized IL-2 matrix was preserved. Cells incubated on the immobilized IL-2 surface could proliferate in response to exogenous soluble IL-2 that was added to the cultures after 36 hours whereas control cultures incubated with an immobilized BSA matrix had died. This indicates that immobilized IL-2 can mediate some of the activity of soluble IL-2 and that internalization of the IL-2 receptor may not be required for at least part of the IL-2 mediated effect.