We compared the persistence of antibacterial activity around antibiotic-impregnated acrylic bone cement discs which were serially transferred on seeded agar plates. On plates inoculated with Staphylococcus aureus ATCC 25923, CMW1 discs containing 2.5% by dry weight of cephalothin, coumermycin or fusidic acid (as diethanolamine fusidate) produced zones of inhibition for four to eight weeks when transferred daily. In contrast, ceftriaxone, cotrimoxazole, rifampicin and vancomycin ceased to be inhibitory within a week. Discs made of 'Palacos-R with Garamycin,' which contains gentamicin 1.25%, had an intermediate duration of activity. When Escherichia coli ATCC 25922 was used as the test organism, ceftriaxone and 'Palacos-R with Garamycin' showed activity for almost three weeks, cephalothin and cotrimoxazole were briefly inhibitory and the remainder not at all. When discs were transferred each week instead of daily, the ranking of antibiotics was similar but antibacterial activity persisted for longer. A combination of gentamicin plus fusidic acid in CMW1 was active for a much shorter time than either fusidic acid alone or 'Palacos-R with Garamycin'. We conclude that coumermycin is a promising new agent for incorporation in acrylic cement.