The plasmid pSC4 which carries a 7.8 kb yeast DNA insert at the BamHI site of the Vector YEp13, complemented simultaneously MO-59-13c lys4, LU75 1ys15 and LU32 lys4lys15 (double) mutations of Saccharomyces cerevisiae. The 1.9 kb BamHl-XbaI DNA insert of the subclone pS051 complemented the LU75 lys15 mutation. The 2.8 kb XhoI-XhoI DNA insert of the pS052 subclone, like pSC4, complemented all three mutations. The 1.9 kb BamHI-XbaI DNA and the 2.8 kb XhoI-XhoI DNA were 100 bp apart in the pSC4 DNA insert and exhibited no homology with each other upon Southern hybridization. The 1.9 kb BamHI-XbaI DNA insert exhibited homology with the pSC4 and pS051 DNA as well as the genomic DNA of MO-59-13c lys4, LU75 lys15, LU32 lys4lys15, and RC1 (LYS) when digested with appropriate restriction enzymes. The 2.8 kb Xhol-XhoI DNA insert exhibited homology with the pSC4 and pS052 DNA as well as MO-59-13c lys4, LU75 lys15, LU32 lys4lys15, and RC1 (LYS) genomic DNA, when digested with XhoI enzyme. The 2.8 kb DNA probe also hybridized with ply(A)+ RNA from RC1 and lys4− transformant but not that from. MO-59-13c lys4 mutant.