The article discusses the quantitation, specificity, and biological relevance of T Cell Growth Factor (TCGF) receptors. TCGF, a 15,000-dalton glycoprotein, is essential for the proliferation of antigen-activated T cells. The study demonstrates that TCGF has characteristics similar to polypeptide hormones, interacting with target cells through specific receptors. The mitogenic effect of TCGF is concentration-dependent and specific to T cells. The binding of radiolabeled TCGF to activated T cells is saturable and specific, with the number of binding sites per cell varying among different cell types. The binding sites are primarily found on TCGF-responsive cells, and the binding is rapid, reversible, and dependent on receptor and lysosome. The concentration of TCGF required for biological activity closely correlates with the concentration needed to occupy the binding sites, suggesting that the biological response is proportional to the occupancy of the receptor sites. The findings support the conclusion that the binding site detected is the receptor through which TCGF exerts its biological effects.The article discusses the quantitation, specificity, and biological relevance of T Cell Growth Factor (TCGF) receptors. TCGF, a 15,000-dalton glycoprotein, is essential for the proliferation of antigen-activated T cells. The study demonstrates that TCGF has characteristics similar to polypeptide hormones, interacting with target cells through specific receptors. The mitogenic effect of TCGF is concentration-dependent and specific to T cells. The binding of radiolabeled TCGF to activated T cells is saturable and specific, with the number of binding sites per cell varying among different cell types. The binding sites are primarily found on TCGF-responsive cells, and the binding is rapid, reversible, and dependent on receptor and lysosome. The concentration of TCGF required for biological activity closely correlates with the concentration needed to occupy the binding sites, suggesting that the biological response is proportional to the occupancy of the receptor sites. The findings support the conclusion that the binding site detected is the receptor through which TCGF exerts its biological effects.