We have isolated the following mutants in the polyamine biosynthetic pathway in yeast: (i) spe10 mutants, which have no ornithine decarboxylase activity and therefore cannot make putrescine; (ii) spe2 mutants, which have no adenosylmethionine decarboxylase and therefore cannot make spermidine or spermine; (iii) spe3 mutants, which have no putrescine aminopropyltransferase and therefore cannot make spermidine and spermine, and (iv) spe4 and spe40 mutants (suppressors of spe10 mutations), which have no spermidine aminopropyltransferase and therefore cannot make spermine. These mutants show that (i) yeast has an absolute requirement for these amines for growth (ii) putrescine in the absence of spermidine and spermine supports growth at one-sixth the wild type rate; (iii) intracellular spermine controls the ornithine decarboxylase activity and thus mutants that cannot make spermine are derepressed for ornithine decarboxylase; (iv) Saccharomyces cerevisiae can make putrescine only by one pathway, i.e., ornithine decarboxylase; (v) spermidine and spermine are synthesized by different aminopropyltransferases in yeast; and (vi) spermidine and/or spermine are absolutely required for both sporulation and maintenance of the double-stranded RNA "killer" plasmid. We have purified ornithine decarboxylase to homogeneity and shown that loss of ornithine decarboxylase activity resulting from growth with added spermidine and spermine is the result of post-translational modification.