Immunoblotting experiments with polyclonal and monoclonal anti-transcription factor IIIA (TFIIIA) antibodies reveal different electrophoretic forms of TFIIIA in extracts from immature and mature oocytes of Xenopus laevis. The well-characterized 39-kD TFIIIA species is present in approximately 10(12) copies per cell in stage I-III previtellogenic oocytes and declines in abundance by 10- to 20-fold during oogenesis. An immunologically related protein of apparent molecular mass of 42 kD is present at 2-4% of the level of 39-kD TFIIIA in immature oocytes, and the level of this protein increases dramatically during oogenesis. Both the 39- and 42-kD proteins are complexed with 5S RNA in 7S ribonucleoprotein (RNP) particles. High-level transcription of the oocyte-type 5S genes in vitro requires 39-kD immature oocyte TFIIIA, whereas both 39-kD TFIIIA and the mature oocyte TFIIIA species of 42 kD support somatic-type 5S transcription. TFIIIA of 42 kD does not support oocyte-type 5S transcription in a fractionated transcription system derived from mature oocytes. Both proteins, however, bind the oocyte-type and somatic-type genes with comparable affinities and exhibit similar DNase footprints on both genes. These results suggest a model for the developmental regulation of 5S RNA gene transcription where 42-kD TFIIIA serves as an activator of somatic-type 5S transcription and as a repressor of oocyte-type transcription during early embryogenesis.