Purines such as hypoxanthine, xanthine, uric acid, allantoin and allantoic acid serve as sole nitrogen sources for the yeast Schizosaccharomyces pombe. A number of classes of mutants unable to use purines have been isolated and genetically analysed. Mutants in the urol gene lack uricase, all1 lack allantoinase, ala1 lack allantoicase whilst in ure1, ure2, ure3 and ure4 genes lack urease activity. Mutants in four hyp genes are unable to convert hypoxanthine to uric acid whilst mutation in xan1 results in impaired growth with xanthine. hyp5 strains are unable to convert both hypoxanthine and xanthine to uric acid. The mutations are recessive and none of the loci are linked to each other. The possible catalytic steps involved are discussed. 