This report describes the in vivo quantitation of neutrophil accumulation at inflammatory sites in rabbits by employing 51Cr-labeled autologous rabbit blood neutrophils. These labeled neutrophils circulated with a half of 3.2 to 3.8 hours. They were found to localize with great specificity at skin sites injected intradermally with zymosan-activated plasma, synthetic chemotactic peptides (e.g., N-formyl methionyl leucyl phenylalanine), Escherichia coli-derived chemotactic factors, or whole E. coli. Contrary to reports utilizing in vitro assays, under in vivo conditions the synthetic chemotactic peptides caused significantly less neutrophil accululation than zymosan-aulation by high concentrations of N-formyl methionyl leucyl phenylalanine was observed. Kinetic studies of the accumulation of labeled leukocytes in the skin in response to intradermal injection of formalin-killed E. coli were performed. Nearly all of the leukocytes accumulated during the first 4 hours after E. coli injection with a peak rate of accumulation between 2 and 3 hours. Essentially no additional localization of leukocytes occurred at lesions which were 6, 8, or 24 hours old. These results demonstrate that 51Cr-labeled rabbit blood neutrophils can be utilized to quantitate the degree of neutrophil accumulation in inflammatory reactions as well as to study the hour by hour kinetics of this accumulation.