The potential usefulness of mecillinam might be diminished if resistant organisms appeared during treatment. Mecillinam resistant phenotypic variants which may be formed in the laboratory have not yet been detected in the faecal and urinary flora of individuals given pivmecillinam. Cross-resistance between meciHinam and ampicillin in faecal and urinary organisms is minimal and a very high proportion of clinical isolates of Enterobacteriaceae are sensitive to mecillinam. The possibility of a slow build up of mecillinam resistance in these organisms cannot at present be excluded. The development of resistance during the prolonged treatment of salmonella infections shows that there is a need to closely monitor the effect of different dose regimes of this antibiotic on the incidence of mecillinam resistance. With these reservations, short courses of pivmecillinam may not be expected to significantly harm the commensal flora or select resistant urinary or faecal Enterobacteriaceae.