A bile acid:3' phosphoadenosine-5'phosphosulfate:sulfotransferase (BAST I) from adult female rat liver cytosol has been purified 157-fold by a two-step isolation procedure. The N-terminal amino acid sequence of the 30,000 subunit has been determined for the first 35 residues. The V(max) of purified BAST I is 18.7 nmol/min per mg protein with N-(3-hydroxy-5β-cholanoyl)glycine (glycolithocholic acid) as substrate, comparable to that of the corresponding purified human BAST (Chen, L-J., and I. H. Segel, 1985. Arch. Biochem. Biophys. 241: 371-379). BAST I activity has a broad pH optimum from 5.5-7.5. Although maximum activity occurs with 5 mM MgCl2, Mg2+ is not essential for BAST I activity. The greatest sulfotransferase activity and the highest substrate affinity is observed with bile acids or steroids that have a steroid nucleus containing a 3β-hydroxy group and a 5-6 double bond or a trans A-B ring junction. These substrates have normal hyperbolic initial velocity curves with substrate inhibition occurring above 5 μM. Of the saturated 5β-bile acids, those with a single 3-hydroxy group are the most active. The addition of a second hydroxy group at the 6- or 7-position eliminates more than 99% of the activity. In contrast, 3α,12α-dihydroxy-5β-cholan-24-oic acid (deoxycholic acid) is an excellent substrate. The initial velocity curves for glycolithocholic and deoxycholic acid conjugates are sigmoidal rather than hyperbolic, suggestive of an allosteric effect. Maximum activity is observed at 80 μM for glycolithocholic acid. All substrates, bile acids and steroids, are inhibited by the 5β-bile acid, 3-keto-5β-cholanoic acid. The data suggest that BAST I is the same protein as hydrosteroid sulfotransferase 2.