We have found that hypertonic medium inhibited the receptor-mediated uptake of the chemotactic peptide N-formylnorleucylleucylphenylalanine without affecting fluid-phase endocytosis by polymorphonuclear leukocytes (PMNs). Morphological and biochemical evidence demonstrated that cells in hypertonic medium did not accumulate peptide in a receptor-mediated manner. However, the cells continued to form endosomes containing fluid-phase markers. Furthermore, the content of these endosomes was processed normally, i.e., both digested and intact material were released into the medium. The inhibition of receptor-mediated uptake was a function of the tonicity. Partial inhibition occurred in 0.45 and 0.6 osmolar medium and maximal inhibition occurred in 0.75 osmolar medium. The inhibition was independent of the solute used to increase the tonicity: sodium chloride, sucrose, and lactose all inhibited uptake to similar extents. Hypertonic medium had little effect on saturable peptide binding. However, it did prevent the clustering of surface molecules as indicated by the inhibition of capping of fluorescent concanavalin A. In addition, hypertonic medium prevented the peptide-stimulated increase in cytosolic calcium levels as measured by quin 2 fluorescence. The tonicity dependence of the inhibition of quin 2 fluorescence paralleled the inhibition of receptor-mediated uptake.