We used site-directed mutagenesis to generate mutations in the -10 region of the lac P2 promoter. The mutations were crossed onto lambda bacteriophage carrying the lac regulatory elements and an intact lacZ gene, and the effects of the various mutations were determined in vivo and in vitro. Two of four mutations had effects on the start point of the P2-directed transcript and had very little effect on lac expression. Another mutation, which abolishes P2 promoter activity in vitro, also had very little effect on lac expression in vivo. We suggest that the P2 promoter plays little or no role in the activation of the P1 promoter by catabolite activator protein in complex with cyclic AMP.