The gam gene of bacteriophage Mu encodes a protein which protects linear double stranded DNA from exonuclease degradation in vitro and in vivo. We purified the Mu gam gene pprduct to apparent homogeneity from cells in which it is over-produced from a plasmid clone. The purified protein is a dimer of identical subunits of 18.9 kd. It can aggregate DNA into large, rapidly sedimenting complexes and is a potent exonuclease inhibitor when bound to DNA. The N-terminal amino acid sequence of the purified protein was determined by automated degradation and the nucleotide sequence of the Mu gam gene is presented to accurately map its position in the Mu genome. © 1986 IRL Press Limited.