Many drugs bind to plasma proteins so that only a fraction of the drug is free in the plasma and capable of producing pharmacological effects. Most drugs bind to albumin, but some - especially basic drugs - bind to α1-acid glycoprotein. Acid drugs (e.g., warfarin or non-steroidal anti-inflammatory drugs) have high affinity for the binding sites of plasma albumin, but some basic drugs (e.g., antidepressants) are bound as well. Most drugs are pharmacologically active at concentrations that do not saturate plasma protein binding sites and thus the fraction of drug bound is independent of the drug concentration. However, the binding sites of a few drugs (e.g., tolbutamide and some sulphonamides) are almost saturated at the therapeutic concentrations. Therefore the application of a higher dose increases the concentration of the free drug by a much greater amount than might be expected. Our TRANSIL technology provides simple assay formats that closely resemble typical physiological protein binding conditions and that are highly reproducible. TRANSIL ready-to-use 96-well microplates are implemented in three fast, reliable and easy assays generating affinity values to the major plasma proteins human serum albumin (HSA), α1-acid glycoprotein (AGP) and rat serum albumin (RSA).