Five men received high-dose, uneven, whole-body gamma-irradiation by accidental exposure to an unshielded 137Cs source. Analysis of the faeces 9-12 days post-irradiation showed low numbers of anaerobes and high counts of enterobacteria and staphylococci in four of the patients and total viable counts of < 10(3)/g in one. All five were treated with systemic ampicillin and gentamicin and oral nystatin commencing 4-7 days after irradiation. Three of the patients were also treated orally with a suspension of an antibiotic-resistant strain of Bifidobacterium longum for 30 days commencing 10-12 days post-irradiation. At 3 weeks post-irradiation, B. longum had appeared in their faecal flora and total anaerobe counts exceeded those of facultative and obligate aerobes. At 4 weeks and 5-7 weeks post-irradiation, this normalisation of the faecal flora continued. In contrast, in the two patients who received a placebo the faecal flora was dominated by enterobacteria (Klebsiella, Enterobacter and Serratia spp.) showing multiple antibiotic resistance 3 weeks post-irradiation. These potential opportunist pathogens were not isolated from the B. longum-treated group. Only one patient in the control group survived beyond 3 weeks; he continued to show high faecal counts of enterobacteria and staphylococci and low counts of obligate anaerobes. 'Probiotic' treatment with this antibiotic-resistant strain of B. longum may be of benefit in the treatment of radiation sickness, aiding normalisation of the faecal flora and inhibiting colonisation and overgrowth with opportunist pathogens.