Oc mutations in the operators of bacteriophage lambda have been used to analyze the functional organization of the operators. In each operator, repressor binding sites 1 and 2, as identified biochemically, were found to be primarily responsible for the repressor affinity of the operators in vitro and for the repression of lytic functions in vivo. In addition, both sites were shown to be involved in the action of cro product at the operators. The data obtained have been used to estimate the repressor affinities of the individual binding sites. These affinities suggest that repressor bound at OR1 and OR2 interacts cooperatively. The results obtained support a model for repression of the early lambda operons where repressor bound at binding sites 1 and 2 interferes with RNA polymerase binding to the promotor sites.