Human T lymphoid cell lines (MOLT-4f, MOLT-3, HSB-2, CEM) and human B lymphoid cell lines (BJAB, RAJI, WIL-2) were grown longterm (up to 8 months) in serum-free medium. This medium consisted of Iscove's modified Dulbecco's medium (IMDM), supplemented with bovine serum albumin (BSA) and transferrin (TF). This serum-free medium containing albumin and transferrin is designated AT-IMDM. Lipids were not essential. Cell viability remained high, greater than 80%, in the serum-free medium and the cells maintained their distinctive characteristics. Interleukin-2 (IL-2) production capacity was maintained by the human T lymphoid cell lines JURKAT-77 and MO in short term culture. This simple medium composed of relatively inexpensive and readily available components should be useful for studies of lymphoid cell growth and differentiation and lymphoid cell products.