Poly(3-hydroxybutyrate) (PHB) quantification has been developed mostly using acidic methanolysis followed by GC analysis of the 3-hydroxybutyrate methyl ester. However, under our experimental conditions, only (62 %) of the ester was detected by GC analysis. Following the study of the different steps involved in this method (i.e., hydrolysis, esterification, and recovery of the ester), the recovery was shown to be limiting. Addition of water to the organic phase, required for its purification before injection, led to the partition of the ester between the organic and the aqueous phase. The influence of the length of acidic methanolysis time on the amount of ester detected was also investigated. NMR analysis was used to show that secondary products were absent in both phases, regardless of heating time. Moreover, increasing acid concentration and the use of lyophilized cells were shown to lead to the decrease of the treatment time. Concerning internal standard choice, methyl benzoate was found to meet all the requirements to correct injection volume errors or to follow organic phase volume changes as a function of acid and water concentrations. The validity of the method was checked on Rhizobium meliloti M5N 1 cells, which are shown to produce about (60 %) PHB (w/w) when cultivated with fructose as the carbon source. ic 1995 Academic Press, Inc.