The program DNAGEL is used to determine the size of DNA fragments run on agarose or polyacrylamide gels. The positions of the bands are read from gel photographs by means of a digitizer. Standard curves are calculated by the method of Southern (1979). The bands, as they are measured, are reproduced on the screen so that erroneous input can be recognized and corrected immediately. Similarly the estimated fragment sizes are printed in a table in the same relative positions as the bands on the gel. This makes it especially easy to relate fragment sizes with the bands on the gel picture. As an additional function the calculated positions of bands can be displayed on the screen. The program DNAGEL is written in APPLESOFT BASIC, suitable for APPLE II computers with 48K memory connected to a monitor, printer and a HOUSTON graphics tablet.