The release of IL-2 binding proteins, derived from the 55,000 MW low affinity IL-2R (L chain), has been observed for virtually all L chain-bearing cells in either humans, the mouse or the rat. Based on the characterization of the released human L chain as a molecule 10,000 MW smaller than the cell surface receptor, either proteolytic cleavage or differential splicing of the L chain encoding mRNA have been suggested as mechanisms underlying the receptor release. Combining affinity labelling of the L chain with [125I]IL-2 and immunoprecipitation with L chain-specific monoclonal antibody (mAb) applied for the detection of soluble rat IL-2R revealed the existence of two classes of soluble receptors, one being of the same size as cell surface-expressed L chain, the other of 40,000 apparent molecular mass. These findings raise the possibility of other mechanisms of receptor release than those discussed for human L chain.