Pivmecillinam, a new penicillin-like antibiotic, is a member of the amidinopenicillanic acid group. Its mode of action differs from that of the classical penicillins and it exhibits no cross-resistance with them. Fifty-two gerontopsychiatric patients, median age 81 years, with E. coli, Klebsiella, and Proteus bacteriurias were divided into three comparable groups. In a ten week open clinical trial the patients were treated with pivmecillinam, pivampicillin or the two drugs given alternately in doses reduced in stages. The bacteriuria in all groups cleared almost completely in three days. Pivmecillinam compared favourably with pivampicillin especially at the end of the reduced medication. The alternating treatment seemed to be superior to treatment with either drug administered separately. No development of resistance was observed. No toxic effect on the liver, kidney, or bone marrow was seen in any of the three groups. In the group receiving pivmecillinam alone, no ampicillin-like skin rashes occurred.