Mutational alteration of the BLM3 gene in Saccharomyces cerevisiae confers hypersensitivities to lethal effects of ionizing radiation, anticancer bleomycins and structurally-related phleomycins. Bleomycin is used clinically in the treatment of many types of cancers, including Kaposi's sarcoma. The BLM3 gene was cloned from a genomic library by complementing the drug hypersensitivities conferred by the codominant blm3-1 mutation. The nucleotide sequence of BLM3 encodes a predicted integral protein of 1804 amino acids with seven to ten potential transmembrane domains and additional motifs. The blm3 null mutation was created by gene replacement, and found not to be essential for growth in the absence of the bleomycin-phleomycin antibiotics. Sequence analyses suggest the Blm3p could be a potential member of the major facilitator superfamily (MFS) of permeases. Northern dot blot analyses using a human RNA master tissue blot containing RNA from fifty different fetal and adult tissues revealed sequence homology in adult tissues to BLM3, but no sequence homology in fetal tissues. The function of the Blm3p is presently unknown. We propose several functions for the Blm3p in protecting cells against oxidative agents, including roles in detoxification, transport and defending against DNA damage.