An accumulated lincomycin intermediate in UC 8292, a lincomycin nonproducing strain of Streptomyces lincolnensis, has been isolated and purified by employing an assay system based on complementation of UC 11066, another lincomycin nonproducing strain of S. lincolnensis. The structure of the purified intermediate is shown to be 3-propylidene-delta 1-pyrroline-5-carboxylic acid, or 1, 2, 3, 6-tetradehydro-propylproline by mass spectrometry and NMR spectroscopic studies. Based on the structure of this newly found intermediate, a biosynthetic pathway for propylproline is proposed as tyrosine-->L-3-hydroxytyrosine (Dopa)-->-->-->-->3-propylidene-delta 1-pyrroline-5-carboxylic acid-->3-propyl-delta 2-pyrroline-5-carboxylic acid-->propylproline.