Plasma transport of 99mTc-p-butyl-IDA was measured by four in vitro methods: trichloroacetic acid precipitation, electrophoresis, HPLC, and Scatchard binding isotherm. The data are in accord with protein transport, the main carrier being albumin with two categories of sites. This work suggests that after IV injection of 99mTc-p-butyl-IDA in humans plasma protein binding is one of the limiting factors for the hepatic deposition of the radiopharmacentical.