Primary astrocyte cultures from spinal cord were purified from contaminating fibroblasts by growth in Dulbecco's modified Eagle's medium (DMEM) in which l-valine was substituted by d-valine. This medium was not supportive for growth of fibroblasts and inhibited their proliferation. The culture purity was assessed using immunofluorescence labelling with specific antibodies to various cell markers. Cultures contained predominantly astrocytes with greater than 92% of this cell type in d-valine medium as opposed to approx. 70% in d,l-valine DMEM medium. This procedure enables primary cultures to be obtained with a larger percentage of astrocytes by a simple modification to the growth medium.