Human pharmacokinetic data on the new anti-parasitic agent, ivermectin, are scanty. For the evaluation of its disposition a specific HPLC assay with sensitive fluorescence detection was developed. Applying equilibrium dialysis, plasma protein binding of ivermectin was measured in five healthy individuals and it averaged 93.2 +/- 4.4% (SD). Such strong binding should be taken into consideration, especially in patients with malnutrition or with diseases in which a decrease in plasma proteins and consequently a higher free fraction of ivermectin could be expected.