A monoclonal anti-I-J k antibody JK10–23 was capable of precipitating the putative I-J k molecule from NP-40 lysates of 125 I-surface labelled mouse T cell clones with either helper or suppressor functions. The I-J molecule detected by specific immunoprecipitation and subsequent one- or two-dimensional gel analysis was a Mr 84–90 K dimer composed of 42–46 K glycopeptide subunits having isoelectric point pH 5.3 to 6.4. A monomeric form of I-J also existed in some of the T cell clones. The I-J subunit was a glycosylated polypeptide with a 41 K backbone having at least two glycosylation sites. I-J was distinguishable from other known dimeric T cell surface molecules with comparable molecular size, that is, T cell receptor αβ heterodimer, A1 and YE molecules expressed on a T cell leukemia EL4, and mouse CD28. The I-J k molecule was precipitable from T cell cl-ones with I-A k and I-E k restriction specificities including a clone derlved from an H-2 b → H-2 bxk F 1 radiation bone marrow chimera. None of the H-2 b -restricted T cell clones from H-2 b and its F 1 showed the I-J k immunoreactivity. T cell clones having either I-A b or I-E k restriction specificities derived from intra-H-2 recombinant mouse B10.A(5R) were positive for the I-J k , while an I-A b -restricted T cell clone from B10.A(3R) was negative in the I-J k Immunoprecipitation. The results indicate that I-J is a novel dimeric surface molecule, most likely to be a homodimer, expressed on T cells according to the major histocompatibility complex.