A modified nonlinear regression analysis program (originally reported by R.G. Duggleby (1981) Anal. Biochem. 110, 9–18) in BASIC for microcomputers and a modified spectrophotometer are described. Their application in the routine determination of concentrations of certain substances by the use of specific enzymes is detailed. Results are given that show the utility of the system for glucose determination by the hexokinase reaction, and its lack of utility for lactate determination using lactate dehydrogenase.