A factor which attracts rabbit polymorphonuclear leukocytes both in vitro and in vivo was isolated from the culture filtrate of a strain of Staphylococcus aureus. The activity of the chemotactic factor was independent of fresh serum and it was nondialyzable. Incubation of the factor with heat-inactivated human serum markedly inhibited its chemotactic property. The factor was heat-labile (80 C, 10 min) in the crude culture filtrate but was heat-stable when partially purified.
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