Xenopus laevis oocytes contain a 38,000-Da protein which serves both as a 5 S gene-specific transcription initiation factor (TFIIIA) and to stabilize 5 S RNA in ribonucleoprotein complexes. Using an antibody to this protein we have measured the levels of TFIIIA during oogenesis and embryonic development in X. laevis. The maximal steady state level (approximately 10(12) molecules/oocyte) is reached early in oogenesis but drops 10- to 20-fold in later stages and another 10- to 20-fold during ovulation. The reduced amount present in the unfertilized egg remains at a nearly constant level throughout early development, but with cell division the cellular concentration drops from 3 X 10(9) to about 10(4) molecules/cell. An immunoreactive protein of the same size is also found in liver tissues and in cultured kidney cells, which also contain about 10(4) molecules/cell. By both structural (CNBr peptide analysis) and functional (transcription of 5 S genes) analyses the embryonic and kidney cell 38,000-Da factors appear indistinguishable from oocyte TFIIIA. In addition a second antigenically related protein of about 40,000 Da is found in late stage embryos, liver tissues, and adult kidney cells (where it is severalfold more abundant than the 38,000-Da TFIIIA). The chromatographic fractionation and functional analysis of the kidney cell extracts has shown that fractions containing the 38,000-Da protein support 5 S RNA synthesis while fractions containing the 40,000-Da protein do not. The significance of these findings for 5 S gene regulation is discussed with respect to the dual function of TFIIIA, the presence of rate-limiting amounts of TFIIIA, and the possibility of stage-specific factors.