Abstract Acidic placental isoferritin inhibited the blastogenic response of peripheral human lymphocytes to T-cell activating lectins. We measured specific binding of radioiodinated placental isoferritin to cells of the T-cell line HD-MAR and found specific high-affinity binding. Binding was faster and more ferritin was bound at 37°C than at 4°C. Displacement experiments indicated that most of the binding occurred at the cell surface. Acidic placental ferritin and isolated H-type ferritin subunits but not isolated L-type subunits, competed for the binding. Scatchard plot analysis showed characteristics of a single binding species with a dissociation constant ( K d) of 1.3–4.4 × 10 −11 M. The results suggest the presence of receptors for acidic isoferritin on T-lymphocytes and thus, a regulatory role for the acidic ferritin H-type subunit in T-cell function.