Recent work with the green alga Dunaliella salina showed the presence of a approximately 20 kDa chloroplast protein that was recognized by polyclonal antibodies raised against the isolated LHC-II [Webb M.R. and Melis A. (1995) Plant Physiol. 107: 885]. In this report, a characterization of the approximately 20 kDa polypeptide is presented. It is shown that it is localized in the chloroplast envelope membrane of D. salina. The abundance of this protein is constant on a per cell basis and independent of the light regime during cell growth. The approximately 20 kDa polypeptide is easily degraded to a approximately 19 kDa product during sample preparation. A limited amino acid sequence of 21 residues from the free N-terminus of the approximately 19 kDa product was obtained. On the basis of this partial sequence, it was concluded that the approximately 20 kDa polypeptide is not a degradation product of a known LHC-II but rather a novel protein. The approximately 20 kDa polypeptide did not cross-react with antibodies raised against the Cbr (carotene biosynthesis-related) gene product and showed a different electrophoretic mobility from the latter. Light-shift experiments suggest that the approximately 20 kDa polypeptide is not an ELIP (early light-inducible protein). Possible functions of the approximately 20 kDa protein are discussed.