(1) UV-induced adenine-less mutants of the ascomycete Ophiostoma multiannulatum (Hedge, et Davids.) v. Arx have been used to elucidate the normal pathway of purine biosynthesis and metabolism in this mould. (2) Investigation of the qualitative growth requirements of different mutant strains provided evidence that the normal pathway of nucleotide synthesis proceeds by way of 4-amino-5-imidazole carboxamide (AICA), hypoxanthine and adenine, or their ribosides or ribotides. (3) This view was supported by the discovery of a mutant type hitherto unknown in Ophiostoma, accumulating AICA riboside and AICA during growth. These mutants are unable to perform the conversion of AICA derivatives to hypoxanthine compounds. Glycine, serine and threonine stimulated the accumulation and consequently are considered as AICA precursors. (4) A comparative quantitative investigation into the efficiency of utilization of the various purine growth substances strongly suggested that the synthesis of nucleotide and nucleic acid adenine and guanine proceeds by way of AICA ribotide, inosine-3-phosphate and adenosine-3-phosphate. (5) Adenosine-3-phosphate was considered as the immediate precursor of nucleic acid purines. A quantitative analysis of growth and of purine metabolism revealed that 89 per cent of the total amount of adenosine-3-phosphate taken up could be accounted for as nucleic acid purines. For adenine this value was only 40 per cent. (6) In the mutants a close relationship exists between the amount of nucleic acids per culture and the production of mycelium, approximately 190 mg. dry matter being formed per of nucleic acid purines. This indicates that in the presence of a given amount of nucleic acid a definite quantity of protein is synthesized.