The morphology and behavior of polymorphonuclear leukocytes (PMNs) were studied after rapid changes in the concentration of a chemotactic factor Nformylnorleucylleucylphenylalanine (f-NorleuLeuPhe) (Schiffmann et al., 1975, Proc. Natl. Acad. Scf. U. S. A. 72:1059-1062). After an increase in peptide concentration, the cells round, form lamellipodia or ruffles over most of their surface, and stop locomotion. These changes are transient. After a delay, the cells, still in the presence of peptide, withdraw most ofthe ruffles and resume locomotion, forming ruffles only at their front. Cells repeat the transient generalized ruffling upon further increase in peptide concentration. The behavioral changes occur over the same dose range as binding to a saturable receptor. The duration ofthe transient response after a concentration increase is roughly proportional to the increase in the numberofcell receptorsoccupied as a resultofthe concentration change. Decreasing the concentration of peptide causes the cells to round transiently and form blebs before they recommence locomotion. The transient natur
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