The transforming proteins encoded by the adenovirus E1A gene bind to a 300-kDa cellular product, p300, via the N-terminal E1A sequences. Residues important for p300 binding are required for the transformation function of E1A and for other E1A-mediated gene-regulating functions, including activation of cell cycle-regulated products and repression of tissue-specific enhancer activity. Recent evidence indicates that p300 is a DNA-binding protein with specific affinity for known enhancer motifs, suggesting that p300 may be a component of transcription factor complexes. The possibility that upstream element-binding factors might interact with basal transcription factors led us to investigate whether p300 interacts, directly or indirectly, with the TATA-binding protein (TBP). We report here that TBP-specific immunoprecipitations show a 300-kDa protein co-precipitating with TBP. This protein is lost from the precipitated material if the lysates are boiled in sodium dodecyl sulfate prior to immunoprecipitation, implying that its presence does not result from non-specific antibody cross-reactivity, but is dependent on specific association with TBP. The TBP-associated 300-kDa protein and p300 originally defined by E1A association show indistinguishable partial proteolytic digest patterns, indicating that these are identical or closely related species. Moreover, p300-specific complexes and TBP-specific complexes include at least two additional common polypeptide species, phosphoproteins of 64 and 59 kDa. These results suggest that p300 interacts with TBP, possibly through intermediate protein-protein associations. They thus provide additional biochemical evidence for postulated protein-protein interactions between upstream regulatory factors and the basal transcriptional machinery.