The relationship between the exposure of Ultrofilm to 5-30-microns slices of 14C-labeled or 125I-labeled brain paste standards was characterized using quantitative autoradiography. After exposing these slices to Ultrofilm for 24, 48 or 72 h, the autoradiograms from the 14C-labeled brain pastes were used to generate standard curves with computerized densitometry. These autoradiographic standard curves were then used to estimate the level of radioactivity present in brain paste slices containing known amounts of 125I, which also had been exposed to the film for the same length of time. The relationship between the calculated concentration of 125I and the radioactivity of 125I as determined by direct gamma counting of slices scraped from slides was a direct linear one. The use of 14C-labeled, as opposed to 125I-labeled, standards obviates the major disadvantage of using 125I-radioligands for quantitative autoradiographic studies.