The invariant gamma chain is transitorily associated with class II histocompatibility antigens during intracellular transport. We have isolated and sequenced a cDNA clone corresponding to the human gamma chain. mRNA hybridizing to the cDNA clone translated into a 33,000-dalton chain that associated specifically with class II antigen alpha and beta chains. The gamma chain consists of 216 amino acids. The two N-linked carbohydrates are attached to asparagines 114 and 120. A continuous stretch of hydrophobic and neutral amino acids occurs in positions 31-56 from the NH2 terminus. This region seems to constitute the transmembrane portion of the polypeptide chain. The positions of the carbohydrate moieties and the putative transmembrane segment indicate that the NH2 terminus of the gamma chain resides on the cytoplasmic side of the membrane. Cell-free translations in conjunction with radiochemical amino acid sequence analyses suggest that the gamma chain lacks an NH2-terminal signal sequence.