The degree of participation (DP) of exogenous thymidine and thymine in overall DNA synthesis was studied in Lactobacillus acidophilus R-26. The DP of thymidine remains constant under a variety of conditions (except at low thymidine concentrations, when the DP is influenced by deoxyribonucleosides and folic acid). A 5-bromodeoxyuridine-resistant mutant was isolated, which displayed cross-resistance to 5-bromouracil and a significantly lower DP of thymidine than the parental strain. Thymine was poorly incorporated in the parental strain even in the presence of deoxyribosides. The results of this investigation would be compatible with the possibility of an alternative pathway for thymidylate synthesis other than the known thymidylate synthase pathway.