Carsiotis, M. (University of Cincinnati College of Medicine, Cincinnati, Ohio), and Ann M. Lacy. Increased activity of tryptophan biosynthetic enzymes in histidine mutants of Neurospora crassa. J. Bacteriol. 89:1472-1477. 1965.-The activities of tryptophan synthetase and indoleglycerol phosphate synthetase, the last two enzymes in the tryptophan biosynthetic pathway, are elevated twofold or more in all histidine mutants of Neurospora crassa. The elevation occurs only when the mutants are grown in a histidine-limited medium. Several possible mechanisms for this elevation were disproved experimentally. The cause of the elevated enzyme activities remains unknown.