The mechanisms underlying the accumulation of polymorphonuclear leukocytes (PMNs) at sites of inflammation are not well understood. The authors developed new methods to evaluate and demonstrate a cell-derived chemotactic factor. They modified the Boyden Millipore system to better distinguish between influences on the rate of PMN locomotion and those on the direction of movement. The methods were used to show that PMNs release substances that stimulate their locomotion and exert true chemotactic effects, even in the absence of serum. The study involved direct morphological evaluation of PMN locomotion and chemotaxis using thin slide cover slip preparations and the Millipore chamber assay. The results demonstrated that PMNs produce a chemotactic factor that stimulates locomotion and chemotaxis, and that this factor is derived from the cells themselves. The study also explored the nature of the chemotactic factor and its requirements for generation and action. The findings provide new insights into the mechanisms of PMN accumulation at sites of inflammation.The mechanisms underlying the accumulation of polymorphonuclear leukocytes (PMNs) at sites of inflammation are not well understood. The authors developed new methods to evaluate and demonstrate a cell-derived chemotactic factor. They modified the Boyden Millipore system to better distinguish between influences on the rate of PMN locomotion and those on the direction of movement. The methods were used to show that PMNs release substances that stimulate their locomotion and exert true chemotactic effects, even in the absence of serum. The study involved direct morphological evaluation of PMN locomotion and chemotaxis using thin slide cover slip preparations and the Millipore chamber assay. The results demonstrated that PMNs produce a chemotactic factor that stimulates locomotion and chemotaxis, and that this factor is derived from the cells themselves. The study also explored the nature of the chemotactic factor and its requirements for generation and action. The findings provide new insights into the mechanisms of PMN accumulation at sites of inflammation.