CNS inflammatory reactions have been implicated in the pathogenesis of several disorders, including Alzheimer's disease, head injury and multiple sclerosis. Activation of brain macrophages, known as microglia, are thought to be an initial event in the pathogenesis of these disorders. An animal model for investigating CNS inflammatory responses has already been developed (Noy et al, this meeting). However, to date there are no automated tools for the quantification of microglia. Therefore, to investigate this link and to assess the therapeutic potential of drug intervention, quantitative approaches need to be developed that can objectively monitor the degree of microglia activation. By developing an automated image analysis technique, we aim to address the issues surrounding the potential involvement of glia in the biological mechanisms underlying the pathogenesis of CNS diseases.Activated microglia in the rat brain were visualised using an antibody to phosphorylated tyrosine residues (P-TYR; Sigma, P3300). Images were captured using a colour CCD video camera and transferred to a Seescan image analysis system. Using a discriminant function analysis paradigm, we have utilised several morphological criteria e.g. circularity, feret diameter, and area/perimeter ratios to generate an automated classification protocol. This method is capable of objectively quantifying the overall P-TYR positive microglia density in the cerebral cortex. This protocol has been validated (>90% accuracy) and details will be presented.The implications of this research are two fold:i) it will provide an accurate method for quantifying activated microglia,ii) its application will be able to definitively address the role of microglia in CNS diseases and assess the impact of potential therapeutic treatments.