We review our methods for definition and phenotypical characterisation of normal and activated rat monocytes. To obtain a comprehensive sample of all blood monocytes including cells from the marginal pool of the blood stream, we extensively perfuse the extrapulmonary circulation with cold PBS/EDTA. Normal rat monocytes are isolated from untreated specified pathogen-free male LEW rats. In vivo activated monocytes are investigated after three days of infusion of recombinant IFN-gamma or during acute renal allograft rejection. Rat monocytes are defined by reactivity with mAbs ED1 and ED9, detecting a lysosomal membrane antigen and a member of the signal-regulatory protein family, respectively, as well as by expression of CD11b. Concomitantly rat monocytes are characterized by the absence of CD5, the absence of the B cell form of CD45R, and the absence of reactivity with mAb RP-1. The majority of the monocytes from untreated LEW rats are CD4+, CD11a(high), CD18high, CD43high, CD62-L-, CD161-, and MHC class II-. Upon stimulation of the immune system in vivo, a second monocyte population increases in number. These cells have a larger diameter and an increased granularity. They are CD4-, CD11a(int), CD18int, CD43low, CD62-L+, CD161int, and MHC class II+. Although some reagents are not yet available (e.g. antibodies against rat CD14 and CD16), rat monocytes can be defined and their state of activation can be characterized. The functionally important population of monocytes, which have already marginated, is accessible by perfusion and relatively high monocyte numbers are isolated per rat. As specified pathogen-free rats are available and numerous experimental systems involving acute or chronic inflammation have been established in rats, differentially activated monocytes may be investigated. The rat is thus a suitable experimental animal for basic research on monocytes.