The presence of high-molecular-weight complexes of aminoacyl-tRNA synthetases in Escherichia coli has been reported (C. L. Harris, J. Bacteriol. 169:2718-2723, 1987). In the current study, Bio-Gel A-5M gel chromatography of 105,000 x g supernatant preparations from E. coli Q13 indicated high molecular weights for both tRNA methylase (300,000) and tRNA sulfurtransferase (450,000). These tRNA modification enzymes did not appear to exist in the same multienzymic complex. On the other hand, 4-thiouridine sulfurtransferase eluted with aminoacyl-tRNA synthetase activity on Bio-Gel A-5M, and both of these activities were cosedimented after further centrifugation of cell supernatants at 160,000 x g for 18 h. Despite this evidence for association of the sulfurtransferase with the synthetase complex, isoleucyl-tRNA synthetase and tRNA sulfurtransferase were totally resolved from each other by DEAE-Sephacel chromatography. Subsequent gel chromatography showed little change in their elution positions on agarose. Hence, either nonspecific aggregation occurred here, or the modification enzymes studied are not members of the aminoacyl-tRNA synthetase complex in E. coli. These findings do suggest that some bacterial tRNA modification enzymes are present in multiprotein complexes of high molecular weight.