A PBS2 phage-coded inhibitor of uracil-DNA glycosylase activity from Bacillus subtilis has been purified extensively and characterized preliminary. The inhibitor has a relative S value of 1.44 +/- 0.08 measured by sedimentation in 15 to 40% glycerol density gradients. It is unusually stable to heating and to the presence of sodium dodecyl sulfate and/or 8 M urea. The inhibitor has no known cofactor requirement and is active in the presence of 10 mM EDTA. Inhibitor activity is sensitive to digestion with proteinase K, but is insensitive to DNase or RNase digestion. The purified inhibitor behaves anomalously during electrophoresis in poly-acrylamide gels containing sodium dodecyl sulfate; however, experiments designed to show that the inhibitor is a glycoprotein were negative. The inhibitor clearly contains a protein required for activity, however, the possibility that some other molecular component is part of the active inhibitor cannot be excluded.