Genes involved in the synthesis of poly(glycerol phosphate) wall teichoic acid have been identified in the tag locus of the model Gram-positive organism Bacillus subtilis 168. The functions of most of these gene products are predictable from sequence similarity to characterized proteins and have provided limited insight into the intracellular synthesis and translocation of wall teichoic acid. Nevertheless, critical steps of poly(glycerol phosphate) teichoic acid polymerization continue to be a puzzle. TagB and TagF, encoded in the tag locus, do not show sequence similarity to characterized proteins. We recently showed that recombinant TagF could catalyze glycerol phosphate polymerization in vitro. Based largely on homology to TagF, the TagB protein has been proposed to catalyze either an intracellular glycerophosphotransfer reaction or the extracellular teichoic acid/peptidoglycan ligation reaction. Here we have taken steps to characterize TagB, particularly through in vivo localization studies and in vitro biochemical assay, in order to make a case for either role in teichoic acid biogenesis. We have shown that TagB associates peripherally with the intracellular face of the cell membrane in vivo. We have also produced recombinant TagB and used it to demonstrate the enzymatic incorporation of labeled glycerol phosphate onto a membrane-bound acceptor. The data collected from this and the accompanying study are strongly supportive of a role for TagB in B. subtilis 168 teichoic acid biogenesis as the CDP-glycerol:N-acetyl-beta-d-mannosaminyl-1,4-N-acetyl-d-glucosaminyldiphosphoundecaprenyl glycerophosphotransferase. Here we use the trivial name "Tag primase."