The polypeptides encoded by the early region of the Mu genome (comprising approximately 8000 base-pairs of DNA extending from the immunity encoding end of the linear Mu genome) have been determined. Minicells were infected by wildtype, suppressor-sensitive-mutation-carrying Mu phages and by λpMu hybrid phages. Comparison of the polypeptides synthesized in these infected minicells with polypeptides synthesized in minicells containing recombinant plasmids, which carry the first 5100 base-pairs of the early region of Mu DNA, have permitted a definitive correlation of the region of DNA with the encoded gene product. The Mu represser protein, gp c ( M r, 26,000), is encoded by approximately 900 base-pairs of DNA extending from the c-end of the genome † † The extreme left end (( c-end or immunity end) of the Mu genome is designated Obp. . The region 1300 to 4300 base-pairs encodes gp A ( M r, 70,000) and gp B ( M r, 33,000) and the region 4300 to 5100 basepairs encodes gp cim ( M r, 7000) and gp kil ( M r, 8000). A 14,000 M r polypeptide, which is probably gp gam, is located in the region 4800 to 5400 base-pairs, and three polypeptides of molecular weights 20,500, 19,000 to 22,000 and 11,500 are located in the region 5200 to 6600 base-pairs. One of these polypeptides is gp sot. The basis for the apparent range of molecular weights (19,000 to 22,000) exhibited by one of the polypeptides encoded in this region is unknown. The region 6200 to 8600 base-pairs encodes three polypeptides ( M r = 10,000, 10,000 and 8000) of unknown function. The polypeptides identified as early polypeptides from their location on the Mu genome are also those polypeptides detected earliest after infection of minicells and are the major translation products of Mu RNA synthesized in minicells infected in the presence of chloramphenicol. These early polypeptides, assuming no overlapping genes, require the expression of approximately 80% of the coding capacity of the early region of the Mu genome.