In vivo chemotaxis of rat leucocytes to N-formyl-L-methionyl-L-leucyl-L-phenylalanine (FMLP).

FMLP was potently chemotactic in vivo to rat leucocytes. Doses of 1 microgram attracted net leucocytes into experimental cotton pellets in 6 h. Maximal chemotactic activity occurred at 0.01 mg. Doses greater or less than 0.01 mg attracted less net leucocytes into experimental pellets. The effect of time on chemotaxis of 0.05 mg FMLP showed that between 3 and 36 h, chemotactic index remained above unity and rose maximally to over 1.5. Maximal chemotactic index occurred at 6 h followed by maximal leucocytic infiltration. FMLP showed early chemoattraction in vivo to rat leucocytes. Leucocytic migration into control pellets rose from 3 h, reached maximum levels at 24 h and remained almost at this level at 36 h. L-methionyl-L-leucyl-L-phenylalanine (MLP), studied as a negative control, showed a chemotactic index and chemotactic differential that rose and fell together between 3 and 36 h. The kinetics of migrating leucocyte populations in response to FMLP showed absolute polymorphonuclear leucocytes at 3 h and over 90% mononuclear leucocytes at 36 h.