NITROGEN METABOLISM IN P. GRISEOFULVUM 289 this fraction. Incorporation into glutamic acid was sufficient to account for the synthesis of at least 85% of the total mycelial ao-amino nitrogen content. The redistribution of label observed after the exhaustion of exogenous nitrogen confirmed previous non-isotopic evidence that a rapid turnover of insoluble nitrogen, involving breakdown to the amino acid level, occurred during nitrogen starvation. Extracellular organic nitrogen released after exhaustion of ammonia arose from the breakdown of insoluble mycelial material. Thanks are due to Mr ID. H. W. Scott for skilled technical assistance in the work describecL in this and the two preceding papers. I am also grateful to Mr R. G. Harrison for carrying out determinations with the mass spectrometer, and to Mr A. F. Henson for help in interpreting the isotopic data.

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