β2-microglobulin is a human serum protein of low molecular weight and unknown function (1). Normally present in serum in low concentrations (2 µg/ml), it is degraded by the kidney (2). In the presence of renal failure serum concentrations as high as 70 µg/ml may occur (3, 4). Patients with defective renal tubular function excrete increased amounts of β2-microglobulin in their urine (4), suggesting that tubular reabsorption is an important step in the catabolism of this protein. In its dependence upon the kidney for degradation, β2-microglobulin is similar to immunoglobulin light chains (5), although microglobulin is not antigenically related to any known immunoglobulin. Recently, Smithies and Poulik have reported the partial amino acid sequence of β2-microglobulin (6). They found significant homology between β2-microglobulin and two adjacent loops (C2 and C3) of the heavy chain of human γG globulin. This finding suggested that β2-microglobulin might be synthesized in lymphoid tissue and prompted us to investigate whether this protein was produced by lymphocytes during in vitro culture.