AbstractThe influence of aging on neutrophil chemotaxis, chemokinesis, and superoxide production was investigated in rats. Animals of two age groups, 3 to 4 months and 20 to 21 months, were used. Equivalent neutrophil chemotactic responses toN‐formyl‐methionyl‐leucyl‐phenylalanine (fMLP), leukotriene B4(LTB4), and bacterial lipopolysaccharide (LPS)‐activated plasma were observed in both groups of animals, with cells suspended in Hanks' balanced salt solution (HBSS). However, cross‐incubation studies in which cells from young adult rats were exposed to plasma from aged donors, then resuspended in HBSS for testing, showed marked changes in the ability of the cells to respond to the chemoattractants. The response to LPS‐activated plasma was reduced, whereas responses to fMLP and LTB4remained unaltered. Previous incubation of the cells with homologous plasma from young donors produced no effect. The inhibitory activity developing with advancing age affected not only chemotaxis but also random movement stimulated by LPS‐activated plasma. The inhibitory activity of chemotaxis and chemokinesis in plasma of aged animals was heat labile (56°), vanished in the presence of a proteolytic enzyme like trypsin, and was maintained after dialysis with 12,000‐Mrretention dialysis tubing. The material did not influence superoxide production by stimulated neutrophils. It is suggested that inhibition of neutrophil locomotion with advancing age is associated with a plasma protein capable of interacting with neutrophil receptors for complement‐derived chemoattractants. The inhibitory substance might influence neutrophil responses to infection and inflammation in the elderly.