The Wilms' tumor 1 gene (WT1) encodes a transcription factor of the zinc-finger family and is homozygously mutated or deleted in a subset of Wilms' tumors. Through alternative mRNA splicing, the gene is expressed as four main polypeptides that differ by a stretch of 17 amino acids just N-terminal of the four zinc-fingers and three amino acids between zinc fingers 3 and 4. We have previously shown that expression of the WT1(-/-) isoform, lacking both inserts, increases the tumor growth rate of the adenovirus-transformed baby rat kidney (AdBRK) cell line 7C3H2, whereas expression of the WT1(-/+) isoform, lacking the 17aa insert, strongly suppresses the tumorigenic phenotype. In the present study we show that expression of these splice variants does not affect the tumorigenic potential of the similar AdBRK cell line, 7C1T1. In contrast to the 7C3H2 cell line, this AdBRK cell line expresses high endogenous levels of EGR-1 (early growth response-1) protein, a transcription factor structurally related to WT1. Ectopic expression of EGR-1 in the 7C3H2 AdBRK cells significantly increases their in vivo growth rate and nullifies the tumor suppressor activity of the WT1(-/+) protein. Furthermore, we find that EGR-1 levels are elevated in some Wilms' tumors. These data are the first to show that EGR-1 overexpression causes enhanced tumor growth and that WT1 and EGR-1 exert antagonizing effects on growth regulation in baby rat kidney cells, which might reflect the situation in some Wilms' tumors.