The phage P22 erf (essential recombination function) gene was placed in a small plasmid under the control of a strong, inducible promoter by manipulations in vitro. Erf protein was purified from induced cells, and characterized. Erf protein (monomer molecular weight 23,000) forms oligomers in solution. The carboxyl terminus is protease-sensitive: its removal generates a discrete amino-terminal fragment (molecular weight approximately 18,000) that also oligomerizes. At temperatures below 45 degrees C, Erf forms stable, discrete complexes with single-stranded DNA and supercoiled DNA, but not with relaxed double-stranded DNA. Binding to single-stranded DNA is stoichiometric; one Erf monomer binds approximately 15 bases of DNA, over the range of protein concentrations tested (2 to 100 micrograms/ml). At high temperatures (50 to 60 degrees C). Erf binds single- and double-stranded DNA, forming aggregates instead of discrete complexes. Heating and cooling in the absence of DNA produces a form of Erf that has single-stranded binding specificity, but forms aggregates on binding.