The E. coli polynucleotide phosphorylase-catalysed reaction of the deoxynucleoside 5'-diphosphates of 5-methyldeoxycytidine, N4-hydroxydeoxycytidine, deoxyuridine and 5-mercurideoxyuridine with the primers d(pT-T-A-G) and d(pT-T-T-T-T-T) have been studied under conditions where the primer is extended, predominantly, by one or two nucleotide residues. In experiments with 5-mercurideoxyuridine 5'-diphosphate, no 5-mercurideoxy-uridine-containing oligonucleotides were produced. The other three nucleotide analogs were found to be good substrates for E. coli PNPase and the conditions established for synthesis with these analogs will allow the construction of a number of biologically useful types of oligodeoxyribonucleotide.