Biotechnology derived medicines will have an increasing impact not only upon medical practice but also upon the working lives of many pharmaceutical scientists. Whilst such medicines may be viewed as highly sophisticated to the clinician and scientist, the computer will still rightly demand that they are both efficacious and safe. Impacting as it does upon all phases of drug development and facilitating quantitative relationship between administered dose and systemic drug concentration, pharmacokinetics has an important role to play in the development of all medicines. Bioanalysis is an essential prelude to any pharmacokinetic investigation. For many biotechnology products the immunoassay and bioassay methodologies employed are often relatively non-specific and imprecise and yield assay dependent pharmacokinetic parameters. Other factors may also confound the pharmacokinetic evaluation of biotechnological products. In vivo binding proteins (including antibodies) may not only interfere with bioanalytical methodology but also have a significant effect on the pharmacokinetics and biological activity of certain macromolecules. Antibody formation is a particular problem in the preclinical evaluation of human proteins. Unlike most conventional pharmaceuticals, the rate and time of delivery into the systemic circulation is a fundamental component of the biological activity of many biological molecules.