A simple program has been developed for calculating areas under concentration-time curves with a home computer. Although it was primarily produced in BBC BASIC for the BBC microcomputer, an adaptation has been made to allow its use with a limited subset of BASIC commands on other computers. After the time and concentration data have been entered, the computer produces a smooth curve running through all data points and employs a spline-fitting interpolation technique to produce equally spaced points, as required by Simpson's rule which is used as the numerical integration procedure. The areas of the vertical strips so formed are then calculated and added together, the integration times being selected by the investigator himself. Comparison with a traditional method showed a good correlation, as did comparison with a commercial NONLIN program. The simple program allows a large number of observations to be analysed quickly, and has proved very useful in calculating AUC values in individual patients in studies with various new antimicrobial agents.