Mutagenesis by 5-bromouracil of lambda phage to clear plaque formers does not depend on the recA function of the host E. coli cell or on the red function of the phage. Pretreatment of the host cells with ultraviolet light does not affect bromouracil mutagenesis of the adsorbed phage. Mutagenesis by hydroxlamine to clear plaque formers takes place at a high level in recA- host cells, and is not changed by preirradiation of of rec+ (wild type) hosts with ultraviolet light. Thus, bromouracil and hydroxylamine appear to mutate lambda phage by a process which differs from that responsible for ultraviolet mutagenesis. Two characteristics of bromouracil mutagenesis--the nonlinear dependence of the number of mutants on bromouracil incorporation, and a high frequency of heterozygotes--fit in with Rydberg's (1977) picture of bromouracil mutagenesis as a consequence of base mispairing, with mismatch repair removing the mutations at low incorporation of the analog.