Phenol sulfotransferase (PST) catalyzes the sulfate conjugation of phenolic and catechol drugs and neurotransmitters. Human platelets and brain contain at least two forms of PST. One form is relatively thermolabile (TL) and catalyzes the sulfate conjugation of monoamines such as dopamine. The other is thermostable (TS) and catalyzes the sulfation of "simple" phenols such as phenol and p-nitrophenol. We found that homogenates of human liver also contain two forms of PST that are similar to brain and platelet TL and TS PST with regard to substrate specificities, thermal stabilities and sensitivities to inhibitors. Optimal conditions were determined for the assay of these two activities in human liver homogenates. The apparent Km of liver homogenate TL PST for dopamine was 27 microM. The apparent Km of the TS form of the enzyme for p-nitrophenol was 0.94 microM. Human liver TS PST also catalyzed the sulfate conjugation of dopamine, but with an apparent Km of 5 mM, over two orders of magnitude higher than that of TL PST. Two different peaks of TS PST activity were separated from the TL activity by ion exchange chromatography of human liver preparations. Both peaks of TS PST activity were partially purified and characterized. Both had similar substrate specificities and inhibitor sensitivities. Km values of TS PST peak I for p-nitrophenol and for 3'-phosphoadenosine-5'-phosphosulfate were 0.91 and 0.86 microM, respectively, while the Km values of TS PST peak II for these two cosubstrates for the reaction were 0.43 and 0.64 microM, respectively. However, the TS PST activity in peak II was significantly more thermolabile than was the activity in peak I. These results are compatible with the conclusion that human liver homogenates contain at least two forms of PST, forms with properties similar to those of TS and TL PST in homogenates of human cerebral cortex and platelets. In addition, human liver contains two isozymes of TS PST.