The biosynthesis of human interleukin 2 receptor (IL-2R)p75 was studied with TU27 monoclonal antibody (TU27 mAb) specific for the IL-2Rp75. TU27 mAb specifically immunoprecipitated not only p75 with a mol. wt of 70-82 kd and an isoelectric point (pl) of 4.4-4.7 but also p70 with a mol. wt of 70 kd and a pl of 4.7 from the cell lysate of MT-2, a human T cell line constitutively expressing IL-2R, labeled metabolically with [35S]cysteine. In contrast, only p75 was detected when the lysate of MT-2 cells surface-labeled with Na125I was immunoprecipitated with TU27 mAb. These results suggest that p75 is a mature form of IL-2Rp75 expressed on the cell surface. Pulse-chase experiments demonstrated that p70 could be converted into IL-2Rp75 by post-translational processing. Studies with monensin, endoglycosidase F, and neuraminidase showed that the mature IL-2Rp75 molecule is generated through N-linked glycosylation and sialylation of the p70 precursor.