It was of interest to examine whether or not a low molecular weight copper-rich metal-thionein was present in biological species which received no metal pretreatment at all. From bovine fetal liver an 8 Cu 2 Zn-thionein having a molecular weight of 11 500 was successfully isolated. 16% of the total copper present in the whole liver were recovered in this protein. During the isolation process anaerobic conditions had to be maintained to avoid uncontrolled oxidation leading to polymeric species and the loss of most of the copper. The similarity of both the present copper-thionein and the polymeric neonatal type mitochondrocuprein was shown. A comparison of different copper-thioneins containing variable amounts of copper was possible when xiCu from 280 nm to longer wavelength was determined. With respect to the ultraviolet properties there were no detectable differences between copper-thioneins prepared either in vivo or in vitro and the fetal copper-thionein. Furthermore, the positions of the Cotton effects as deduced from circular dichroism measurements were rather similar although the magnitude of the observed Cotton extrema was less pronounced and sometimes the signs were reversed. X-ray photoelectron spectrometric studies revealed a Cu(2p3/2) binding energy value of 932.9 eV. Unlike the S(2p1/2,3/2) value near 162 eV using Cu-thioneins from chicken liver or yeast the higher S(20p1/2,3/2) binding energy of 163.0 eV employing fetal Cu-thionein was attributed to partial oxidation of the protein moiety and/or a particular chemical environment. The second S(2p1/2,3/2) peak was assigned to the copper catalyzed oxidation of sulphur via OH to yield RSO-3. In the X-ray photoelectron spectrum of the apoprotein one homogeneous S(2p1/2,3/2) band at 163.7 eV was seen attributable to RSSR.