Microglia, the resident macrophages of the brain, are monocytic cells whose phenotype is determined during development by the unique environment of the central nervous system (CNS). They are quiescent cells when compared with other tissue macrophages, and this downregulation may have important consequences for inflammatory and immune responses in the brain. In the search for features of the brain environment which might exert an influence on microglial behaviour, we have concentrated on the possible role of adhesion molecules. We have developed a robust and reproducible in vitro adhesion assay to look at the interaction between macrophages and brain tissue. We describe here the characterisation of this assay. By injecting agents into the brain in vivo, we were able to study the effect of perturbations in the resident cell population on the adhesion of macrophages to brain tissue in vitro. This provided strong evidence that RAW 264 cells adhere to neurones in preference to other CNS cell types in this assay, and this was confirmed by adhesion assays performed on monolayers of individual cell types. We hypothesise from these results that macrophages interact with CNS neurones in vivo via adhesion molecules, enabling them to sense and respond rapidly to pathology in the brain.