Insulin receptors have been demonstrated on activated but not resting T-lymphocytes. We compared insulin binding to several T- and B-cell tissue culture lines transformed by either radiation or lymphotrophic Herpes virus. Two T-cell lines derived from mouse lymphomas, one T-cell derived from the cotton top marmoset and one mouse B-cell line bound, at most, 3% of a tracer dose of [125I]insulin. In contrast, positive B-cell line (1605 L) derived from a leukemic cotton top marmoset, bound 40% of tracer [125I]insulin. The binding was of high affinity (1.9 nM-1), displayed analogue specificity characteristic of the insulin receptor and receptor number per cell was 16,600. The data show that a transformed B-lymphocyte binds substantial levels of insulin. The magnitude of such expression might be related to the fact that this B-cell line also expresses Epstein-Barr virus.