1. The plasma protein binding characteristics of ceftriaxone, a new cephalosporin antibiotic, were determined in human, baboon, rabbit, dog and rat plasma. 2. The protein binding of ceftriaxone was similar and concentration-dependent in human, baboon, rabbit and rat plasma, being highly bound (90-95%) at low concentrations (less than 100 micrograms/ml) but considerably less bound (approx. 60%) at high concentrations (greater than 400 micrograms/ml). Binding in dog plasma was also concentration-dependent but much lower (approx. 25%) at lower concentrations (30 micrograms/ml) and virtually unbound (2%) at high concentrations (1 mg/ml) over a similar concentration range. 3. Binding of ceftriaxone to human plasma involved two sites: a high affinity-low capacity (saturable) site and a low affinity-high capacity site. Binding to dog plasma apparently was at a single, high affinity-low capacity site. 4. The pharmacokinetics of ceftriaxone in an animal species with binding characteristics similar to man (baboon), appear to be non-linear when based on total drug concentration and linear when based on the free drug concentration. In the dog, pharmacokinetic parameters did not change appreciably if calculated from total or free drug concentrations, due to the low protein binding.