Samples of ventricular cerebrospinal fluid, tumor cyst fluid, and subdural fluid were obtained from 30 patients at operation. The protein concentration and the specific gravity of each sample were measured and the corresponding mean absorption numbers were calculated from the numerical printout of the preoperative computerized tomogram. For fluids with a specific gravity greater than 1.005, a linear relationship was demonstrated between protein concentration and specific gravity. For protein concentrations greater than 300 mg/dl, there was a linear relationship between protein concentration and the mean absorption number. As the precision of present instrumentation improves, it is expected that a noninvasive technique for estimating intracranial protein concentration will have a number of clinical applications.