Estrogen sulphotransferase plays a major role in controlling intracellular levels of 17 beta-estradiol in human mammary cancer cells and human endometrium. Bovine estrogen sulphotransferase c-DNA has recently been cloned; the encoded protein having a maximum Mr of 35,000 (Nash, A.R. et al. (1988) Aust. J. Biol. Sci. 41, 507-516). Enzyme of Mr 35,000 by SDS-PAGE has now been isolated and cyanogen bromide-cleaved peptides sequenced. The latter were identified in the c-DNA-predicted amino acid sequence which confirms that the active enzyme (Mr approximately 70,000) exists as a dimer of identical subunits. Sequence data on similar peptides isolated from an enzyme preparation containing a protein of Mr 74,000 as the major species on SDS-PAGE, which was previously thought to represent the enzyme, suggested that this protein was transferrin. This was confirmed by PAGE, SDS-PAGE, susceptibility to neuraminidase and reaction with bovine transferrin antibody. Isoelectric focusing experiments show that active enzyme exists in two or three polymorphic forms (pI values 5.3, 5.7 and possibly 5.9) having similar physicochemical properties of polymorphic forms of transferrin so that they overlap on ion-exchange chromatography and PAGE. The enzyme shows some homology to the amino acid sequence close to the Fe-binding site in lactoferrin and the question is raised as to the possible presence of a tightly bound metal in estrogen sulphotransferase involved in the binding of adenosine 3'-phosphate 5'-phosphosulphate.