To determine whether blocking a degradative pathway leads to amino acid accumulation, the internal free concentrations of threonine, methionine, and related amino acids have been measured in a strain of Saccharomyces cerevisiae and its mutants lacking either the L-threonine deaminase enzyme (EC 4.2.1.16) coded for by the ILV1 gene (ilv(-)mutants), that coded for by the CHA1 gene (cha(-)mutants), or both threonine deaminase enzymes (ilv(-)cha(-)mutants). Whereas maximal accumulation of internal free amino acids occurred in the double mutants ilv(-)cha(-), ilv(-) single mutants displayed amino acid concentrations higher than those of either the wild type or the cha(-) single mutant. On the other hand, when these enzymes were measured in threonine and methionine overproducer mutants of an industrial strain of Saccharomyces cerevisiae, results indicated, in most cases, lower activities than those of the wild type, although there was not a total correlation between maximal threonine and/or methionine accumulation and minimal enzymatic activities. Results point to the isolation of strains that accumulate essential amino acids by blocking their degradative pathway.