Modified nucleosides can be prepared by microbial transglycosylation from cheaper nucleoside precursors using free or immobilised whole cells. An efficient screening method to find transglycosylation activity in microorganisms was developed for the synthesis of 6-modified purine nucleosides, such as 6-chloro-, 6-methoxy-, 6-iodo-and 6-mercaptopurine ribonucleoside. Out of 100 microorganisms screened, Bacillus stearothermophilus ATCC 12980 was the best for this purpose.