Tuftsin (Thr-Lys-Pro-Arg) is a natural immunomodulating peptide. We have investigated for the presence of a specific tuftsin receptor on murine Kupffer cells using fluorescein-labeled tuftsin, which retains full biological activity. After incubation with fluorescein-labeled tuftsin, Kupffer cells displayed clear binding of this compound on the plasma membrane. Excess tuftsin inhibited this binding, indicating the presence of specific tuftsin receptors on the Kupffer cells. We then investigated the effect of tuftsin on the phagocytic activity of these cells. Phagocytosis assays were performed on 24-well plates between murine Kupffer cells and fluorescent microspheres. The greatest stimulatory effect of tuftsin on percent phagocytic cells over the control value was observed when the cells were incubated with particles at 1 microgram/ml tuftsin for 15 min at 37 degrees C with a particle-to-Kupffer cell ratio of 50:1. Tuftsin also markedly increased the number of particles engulfed by Kupffer cells under the same conditions. These results indicate that Kupffer cells have specific tuftsin receptors; thus tuftsin can stimulate phagocytic activity of Kupffer cells, which constitute the majority of macrophages in the host and are situated strategically in the liver for host defense.