The sera of two patients with autoimmune disorders recognize in HeLa cell extracts two proteins with apparent molecular masses of 37,000 (p37) daltons and 32,000 daltons (p32). These proteins are non covalently associated with 5S RNA and sediment as 7-10 S particles in sucrose density gradients. Both proteins are antigenetically related to TFIIIA, a previously described protein of Xenopus laevis, which is known as a 5S RNA transcription factor and occurs in oocytes as a noncovalent complex with 5S RNA. Like TFIIIA, HeLa cell proteins p37 binds in vitro to 5S RNA and to cloned 5S RNA genes. These results suggest that protein p37 fulfils in HeLa cells a function similar to that of TFIIIA in amphibian oocytes, ie control of 5S RNA transcription.