mRNA obtained from green leaves of lentil (Lens culinaris) was used to construct a cDNA library in phage lambdagt11. The cDNA library was screened with antibodies raised against lentil glycolate oxidase and catalase. Clone CL 1 containing the full-length sequence complementary to glycolate oxidase mRNA was characterized and sequenced. In addition, a 800-base pair catalase cDNA clone was sequenced. To prove the correlation of cDNA insert in CL 1 with glycolate oxidase, the cDNA was transcribed in vitro. The mRNA was translated in vitro yielding a 43 kilodalton protein immunoprecipitable with anti-glycolate oxidase serum. Nucleotide sequences of lentil cDNA and spinach cDNA were 86% identical. Lentil glycolate oxidase was characterized by a C-terminal sequence -P-R-A-L-P-R-L. The expression of glycolate oxidase mRNA in cotyledons, leaves and roots was compared with that of catalase. In leaves, the relative amount of glycolate oxidase mRNA increased during the first 2 days of greening, but decreased later, and was hardly detectable during senescence. In cotyledons of germinating seeds, the level of glycolate oxidase mRNA was markedly lower than the catalase mRNA.