[¹²⁵I] Labeled thyrotropin binding to leukocytes has been studied using lymphocytemonocyte- and polymorphonuclear leukocyte-enriched preparations obtained by centrifugation on Ficoll-Angiocontrix gradients and Sephadex G-10 adherence. From the relation between thyrotropin binding and phagocytosis as shown by latex beads ingestion, it is concluded that the hormone binds essentially to monocytes and polymorphonuclear leukocytes. Equilibrium association constants of the high-affinity, low-capacity sites (1 nM⁻¹) are similar to those found in isolated thyroid cells or in thyroid plasma membranes. The role of thyrotropin in the regulation of phagocytosis by leukocytes is discussed.