In the presence of high concentrations of any of several types of macromolecules, DNA ligase preparations from rat liver nuclei or from Escherichia coli actively catalyze the blunt-end ligation of DNA. This is in contrast to the lack of activity on such substrates by these enzymes under conventional assay conditions. In addition, the previously established activity of T4 DNA ligase on blunt-ended molecules is greatly increased in the presence of high concentrations of macromolecules. Because such crowded solutions may well be a more adequate model for intracellular conditions than assays in dilute solutions, we suggest that blunt-end ligation may be a widely occurring reaction in vivo.