Suspensions of Escherichia coli and Klebsiella aerogenes were exposed to low levels of chlorhexidine, which had little effect on their in vitro viability. A substantial reduction in their in vivo infectivity measured by intraperitoneal inoculation of mice was found. This selective reduction in bacterial infectivity by chlorhexidine should be taken into account when conventional in vitro tests are used to assess the clinical effectiveness of chlorhexidine-containing antiseptics.