SAGA (Spt-Ada-Gen5 histone acetyltransferase complex) is a histoneacetylase complex which has Gcn5p as catalyst subunit and functions overlapping with the fundamental transcription factor TFIID which has Taf1p as catalyst subunit. However, SAGA and TFIID have different allotment and each accomplishes the important role in the transcription for Housekeeping gene group and Stress Responding gene group. Also, SAGA is a large protein complex which is composed of Ada protein group (5 kinds), TBP related protein group (4 kinds), TAF protein group (5 kinds that also pertain to TFIID), and other protein groups (>6 kinds). As its molecular function, there is the chemical modification of histone, or the recruitment by direct interaction of transcription regulating factor on DNA, or the control of transcription initiating reaction by TBP. Spt3p is one kind of the above-mentioned TBP related protein group and in the case of budding yeast it is composed of 337 amino acid residues (aa). The product is prepared by immunizing rabbit with recombinant protein which was over-expressed in E. coli with a plasmid carrying the N terminal domain (1-200aa) of Spt3p protein of budding yeast, and purified by chromatography. Using the product as antiserum in Western blotting, the band of 45 kD pertaining to Spt3p was obtained from the extract solution of budding cells.