Murine T and B lymphocytes can be induced to release soluble interleukin 2 receptors (IL2Rs). This receptor is believed to be a truncated form of the 55-kDa chain of the cell-membrane-associated receptor. It has been speculated that this receptor may play an important immunoregulatory role by binding to interleukin 2 (IL-2). We report here that interleukin 5 can induce normal murine B cells to release soluble IL2Rs. This extends our finding that interleukin 5 similarly can induce murine B cells to express functional cell-surface-associated IL2Rs. Two possible mechanisms of release of soluble IL2Rs have been suggested. Soluble IL2R could be synthesized as a secretory form of the receptor lacking the transmembrane domain or by cleavage of the extracellular domain of the cell-surface-associated IL2R at the cell surface. To investigate which mechanism was operative, we radioiodinated the cell surface of normal murine splenocytes that had been cultured for 1 day with Con A to stimulate the expression of cell-surface-associated IL2Rs and the release of soluble IL2Rs. Under the conditions used, radiolabeling of internal proteins was not apparent. Labeled cells were then recultured with Con A, conditioned medium was taken from replicate cultures at various times after radioiodination, and the specific radioactivity of released soluble IL2Rs was determined by ELISA and RIA. We demonstrate that the specific radioactivity and the kinetics of change of the specific radioactivity are consistent with the hypothesis that the soluble IL2Rs are derived from the cell-surface-associated IL2Rs rather than being released in a secretory form.