This paper reports an experimental investigation of the extent to which focal spot size influences image resolution in mammography. Films of two different phantoms, both containing high contrast bar patterns, were obtained using three different foci from 0.9 to 0.1 mm, and three degrees of magnification with the smallest of those foci. Some influence of focal spot size was detectable in all films except for the 0.1 mm focus at low magnification, where it was negligible in comparison with film-screen unsharpness, but effects attributed to the film-screen system were more important than focus size except with the 0.9 mm focus or at high magnification. However, a nominal 0.4 mm focus may have an effective size of 0.9 mm close to the chest wall side of the film. These two phantoms, and two others, were used to investigate the changes of perceptibility of "realistic" details, all of which depend on a combination of contrast, resolution and noise, with changes of focal spot size and magnification. As expected, when judged in this way image quality improved as focal spot size decreased. With the fine focus it also improved as magnification increased, unlike changes in high contrast resolution which decreased for high magnification. Thus conventional bar patterns are not always a good guide to detail perceptibility in mammograms, where the effects of noise may be as important as those of focal spot size.