Abstract: Complementary DNA encoding a 52 kDa form of a protein present in the human Ro/SSA ribonucleoprotein complex has been cloned. A lambda gt11 cDNA library made from human thymocyte mRNA was screened with serum from a SLE patient and two immunoreactive clones were isolated. These clones reacted with other patient sera which had anti-52 kDa Ro/SSA antibodies and with affinity purified anti-52 kDa Ro/SSA antibodies. Moreover, affinity purified antibodies eluted from fusion proteins of the isolated clones reacted only with the 52 kDa protein of lymphocytes in the Western blot. Ro/SSA RNAs were also precipitated with these affinity purified antibodies, further confirming that the clones encode a 52 kDa Ro/SSA antigen. The sequence differs from the previously reported 60 kDa Ro/SSA gene.