The interaction of the synthetic serum thymic factor (FTS, "facteur thymique sérique") with human T lymphocytes from two established T cell lines (1301 and HSB2) was studied by using [3H]FTS. At 30 degrees C, in the presence of 10 nM labeled FTS, a steady state was observed within 120 min. Bound labeled FTS was dissociated by simultaneous addition of an excess of unlabeled FTS (dissociation constant Kd = 3 nM). The binding of [3H]FTS was reduced to 10-20% when unlabeled FTS was added at 20 microgram/ml (20 micro M), but not when unrelated peptides or FTS analogues differing from FTS by only one amino acid residue were added. No FTS binding was detectable on B and null cell lines or on two T cell lines other than 1301 and HSB2. These data indicate that only certain T cell subsets possess receptors with high affinity and specificity for FTS.