Delayed hypersensitivity reactions to a battery of antigens were measured in 145 Tunisian breast cancer patients to determine whether an immunologic mechanism could be detected which might explain the high frequency (60%) of the rapidly progressing form in Tunisian breast cancer patients. Although a greater proportion (30%) of patients with rapily progressing breast cancer reacted to extracts of a breast tumor antigen (2937) than patients without PEV (9%), no significnat difference between PEV and non-PEV patients could be found in reactivity to DNCB, standard microbial antigens, or extracts from tissue culture cell lines. Rapidly progressing breast cancer in Tunisia is not associated with an impairment of delayed hypersensitivity.