A gene bank of partial Sau3A restriction fragments of S. pombe DNA has been constructed in the plasmid vector, pDB248', which is capable of high frequency transformation of S. pombe. Procedures are described which enable plasmids to be recovered from S. pombe by their reintroduction into E. coli. These methods have been used to detect the S. pombe genes lys 1+, ade 6+ and his 2+ in the gene bank by complementation of mutant gene functions, and to physically isolate the lys 1+ gene.