A menu-driven program--written in C and Assembly language--has been developed for use with an IBM PC or compatible computer for calibration of ion-selective microelectrodes. Available features include (i) user-logical commands, (ii) rapid, accurate construction and display of calibration curves (fitted by least squares non-linear regression), (iii) analysis of data (stored either on- or off-line), with correction for drift and conversion from voltage or length to ion concentration or activity, (iv) calculation of selectivity coefficients, (v) optional storage and use of activity coefficients, and (vi) output of data, calculations and graphics to printer and plotter. A simple chamber and valve-activated flow-system for rapid change of solutions with either manual selection or computer control is also described.