Concentrations of penicillin, doxycycline, and ciproftoxacin were measured by bioassay in sera of rhesus monkeys treated with these drugs for inhalation anthrax. Antibiotic doses were determined on the basis of published serum concentration data from humans and comparative body surface area calculations for humans and rhesus monkeys. The antibiotics were well tolerated. Serum peak and trough concentrations of penicillin, doxycycline, and ciproftoxacin, respectively, averaged 2.7 and 0.8, 1.31 and 0.26, and 1.22 and 0.14 µg/mL. These were within the range usually observed with standard oral doses in humans, and peak concentrations in all monkeys exceeded the MICs for 90% of Bacillus anthracis strains.