Our knowledge of the response of blood leucocytes from normal animals to bacteria and their products has been stimulated in recent years by development of techniques permitting greater understanding and a more quantitative treatment of the process (Martin et al, 1950; Miles and Niven, 1950). The ability of certain organisms to inhibit the migration of leucocytes in vitro has also been correlated in some cases with the production of endotoxins by the organisms (Martin and Chaudhuri, 1952) and with the formation of cell substances apparently associated with the quality of virulence. The objectives of the present study have been to examine the phenomenon of leucocyte migration in response to brucellae and some of their products, to extend the study to leucocytes derived from animals infected with brucellae and other organisms, and finally to determine whether the leucocytes of infected animals or of animals treated in other ways are inherently affected so far as migration capacity is concerned by changes impressed upon the host or whether the leucocytes are merely indirect indicators of plasma alterations.