A set of plasmids containing portions of the Col E1 plasmid were transformed into recA- cells. These cells, after UV irradiation, only incorporate labelled amino acids into plasmid-encoded proteins. UV-irradiated cells label a 14.5 kDa band if they are phenotypically immune to colicin E1, and do not contain this band if they are sensitive to colicin E1. We conclude that the 14.5 kDa protein is the colicin E1 immunity protein. When the inner and outer membranes of these cells are fractionated, the labelled band appears in the inner membrane. The immunity protein must be an intrinsic inner membrane protein, confirming the predictions made by hydrophobicity calculations from primary sequence data.