The bacteriophage lambda repressor, a protein that maintains the lysogenic state of a bacterium containing a lambda prophage, is cleaved when the lysogen is induced by mitomycin C or ultraviolet light. This cleavage does not occur when induction is prevented by mutational alteration either of the phage repressor or of the host recA gene product. Proteolytic cleavage may be the primary mechanism of repressor inactivation in this induction pathway, or it may follow a different event which causes the initial inactivation.