The Wilms' tumor susceptibility gene, WT1, is altered in a subset of Wilms' tumors and encodes a transcription factor with four zinc fingers. Here we describe the isolation of native WT1 protein from frozen normal human kidney and Wilms' tumor samples. Through size exclusion chromatography and Western blot analysis we determined the elution pattern of WT1. The majority of WT1 from adult kidney and Wilms' tumor specimens was found to elute at a size of approximately 120 kDa, consistent with a WT1 homodimer and some WT1 protein was also found in a higher molecular weight complex. In 14 week fetal kidney the majority of the WT1 protein eluted at a size of 80 kDa, suggesting that at this developmental stage the WT1 protein is not present as a homodimer. The identity of complexing partners can now be studied using this approach.