Abstract A protein of molecular weight 71,000 (P 71) was synthesized by cells associated with the cerebral microvasculature in telencephalon slices incubated in vitro. This protein, however, was not synthesized in vivo. There was a 30 min delay in the synthesis of P 71 by brain slices. During this lag period a new ribonucleic acid species which was required for P 71 synthesis was produced in the slices. After this delay P 71 became a major product of protein synthesis. P 71 was also produced by microvasculature enriched fractions isolated from telencephalon. This protein may be involved in the initial response of the vascular system to injury.