Most work reporting the sterol composition of living organisms has not been done quantitatively, although good quantitative data are available for fatty acids and many other cellular components using an internal-standard method that compensates for errors during gas chromatographic analysis. In this paper, we report on the use of 7-stigmastenyl acetate as an internal standard for sterol analysis in two species of phytoplankton and oysters produced with two different diets. This internal-standard method provides an internal standard for this entire process of analysis, not just the gas chromatographic analysis. When analyzing 50-microgram samples of cholesterol acetate after hydrolysis and acetylation, about 30% of the sample was lost, resulting in a 30% error using the older external-standard method. Using the internal-standard method, the analysis error was less than 2%. Losses of sterol during analysis apparently are greater with plant and animal samples than with pure sterol standards. This internal-standard method was shown to be extremely useful, especially for samples with less than 500 micrograms of sterol. Finally, the standard error in sterol analysis is much lower when the internal-standard method is used, allowing statistical distinctions that are not possible otherwise. Use of 7-stigmastenyl acetate as an internal standard offers several advantages over the use of cholestane.