The basic concepts underlying the theory and experimental determination of receiver operating characteristic (ROC) curves are discussed. Such curves were used to describe the detectability of the image of 2 mm Lucite beads (similar to certain small gallstones) in a noisy background of radiographic mottle. Results are shown for four typical radiographic screen-film combinations with differing optical, sensitometric, and noise properties. In some cases there was qualitative correlation between the detectability described by the ROC curves and the mathematical model describing radiographic mottle.