A method of combining two separate regressions,YonxandYoonz, to yield confidence intervals forx's corresponding to given values ofz's, is illustrated in the following chemical calibration problem: A chemical indicatorz, (oxalacetic acid), called the standard, is used to estimate the average amount of serum glutamic oxalacetic transaminase (SGOT) enzymexpresent by in effect comparing the regressions ofYonxandYoonz, whereYandYoare their respective light absorbences. It is shown that single and joint contidence intervals for expected enzyme levels corresponding to indicator values are given by the solution of quartic equations when (as for this particular enzyme) the regression ofYonxis (approximately) quadratic and the regression ofYoonzis linear (and both are through the origin).