A rapid method is presented for the direct qualitative and quantitative assay by gas chromatography of microbial metabolites produced in culture and biological specimens. We have used this technique for analysis of metabolic end-products of anaerobic bacteria in broth cultures and in feces, but it would be suitable for studies with other organisms and biological specimens as well. Aqueous samples are injected directly into the system. Two gas chromatograph instruments are connected to one recorder, and employ thermal conductivity (TCD) and flame ionization detectors (FID). The column packings (Resoflex LAC-1-R-296 standard concentration (P) 1 1 Burrell Corporation, 5th Avenue, Pittsburgh, PA 15219. for the TCD and 6% FFAP 2 2 Varian Aerograph, Walnut Creek, CA. FFAP is a reaction product between Carbowax 20M and 2-nitroteraphthalic acid. on Porapak Q 3 3 Waters Associates, Framingham, MA 01701. for the FID) permit separation of lower-chain volatile fatty acids (VFA's), alchohols and similar components as well as methyl ester derivatives of nonvolatile acids (lactic, pyruvic, and succinic) in a comparatively short time. Quantitation of the VFA's is provided by means of an electronic digital integrator.