The biochemical lesion in two cysteine auxotrophs of Tetrahymena thermophila has been established as a defect in S-adenosylhomocysteine hydrolase, an enzyme of the transsulfuration pathway. As a result, these mutants require cysteine (or cystathionine or homocysteine) for growth in a defined medium. Cell-free extracts of the mutants contained less than 5% of the level of the enzyme seen in the wild type. One of the mutant strains accumulated intracellular levels of S-adenosylhomocysteine as high as 1380 microM, a level 200 times normal. When both mutant strains were maintained in defined medium without cysteine, growth occurred after a long lag; this phenomenon was termed "adaptation." Adaptation was a) reversed by passage through rich medium, b) was not a recovery of S-adenosylhomocysteine hydrolase, and c) was probably linked to induction of an alternate pathway for cysteine biosynthesis, involving a lysosomal S-adenosylhomocysteine nucleosidase activity.