A computer program, HCNO-MAP, has been specially designed to evaluate data from the analysis of biological samples done with a scanning nuclear microprobe. It produces quantitative maps of the elements 1H, 12C, 14N and 16O from spectra of elastically backscattered and forward-scattered protons, at a speed of 0.3-0.4 s/pixel on a microVAX-II computer. Algorithms for the automatic evaluation used in the program are described. Results from analyses of plastic foils show that the program is capable of handling poor pulse statistics. A quantitative map of 12C in a cross section of a rootlet is presented.