To study the ecological effects of pivmecillinam on the human oropharyngeal, intestinal and skin microflora, 15 healthy volunteers were given pivmecillinam tablets 400 mg twice daily for 7 days. Saliva, stool and skin specimens were taken before (days -3 and 0) and on the 2nd, 4th and 7th days during the administration period and 14 and 21 days after the start of administration. Mecillinam caused no major changes in the aerobic or anaerobic oropharyngeal microflora. In the aerobic intestinal microflora there was a decrease in the numbers of Escherichia coli while no changes occurred in the anaerobic microflora. In the skin microflora there was a transient decrease in the numbers of Propionibacterium spp. underneath the wing of the nose. The major effect of pivmecillinam was seen on E. coli and to some extent on Propionibacterium spp. No further ecological disturbances were noticed in the oropharyngeal, intestinal or skin microflora.