Increased expression of the transcription factor Spi-1 (PU.1) results from retroviral insertion in nearly all Friend spleen focus-forming virus-transformed murine erythroleukemia cell lines and exposure of these cells to Me2SO, induces their differentiation and decreases Spi-1 mRNA level by 4-5-fold. While these results suggest that alterations in Spi-1 expression have significant effects on erythroblast growth and differentiation, neither the cause nor the effect of the decrease in Spi-1 expression that follows Me2SO exposure has been established. The experiments described here demonstrate that the effect of inducers on Spi-1 expression is regulated post-transcriptionally. Nuclear run-off transcriptions demonstrated that Spi-1 transcription was not decreased following Me2SO exposure. Additionally, expression of a recombinant Spi-1 mRNA under transcriptional control of a constitutively active Rous sarcoma virus promoter was regulated identically to endogenous Spi-1 mRNA. The ability of Me2SO to destabilize Spi-1 mRNA was selective, as the stability of the erythroid transcription factors GATA-1 and NF-E2 were not similarly effected. The effect of Me2SO on the stability of Spi-1 mRNA provides a novel means of altering gene expression in these cells and is likely to have significance for the differentiation of these cells.