Class II transactivator (CIITA) is necessary for expression of class II MHC (MHC-II) molecules. In mice, CIITA expression is regulated by three promoters (pI, pIII, and pIV), producing types I, III, and IV CIITA. The relative roles of different CIITA types remain unclear. Unstimulated bone marrow-derived macrophages expressed low levels of CIITA mRNA; type I CIITA was nine times more abundant than type IV (type III CIITA was barely detected). Exposure to IFN-gamma (6 h) dramatically increased types I and IV CIITA mRNA to similar absolute levels. Type IV CIITA declined over time, but type I was stable for over 72 h. Thus, the dominant form of CIITA evolved with time during activation by IFN-gamma, and type I CIITA explained prolonged expression of MHC-II by macrophages. mRNA half-life was shorter for type I than type IV CIITA, suggesting that sustained transcription contributed to stable expression of type I CIITA induced by IFN-gamma. Splenic B cells expressed mRNA for type III CIITA but very little for types I or IV. Treatment with IL-4 increased surface expression of MHC-II protein, but mRNA for MHC-II and CIITA (total, I, III, and IV) remained unchanged, suggesting posttranslational regulation. Splenic dendritic cells expressed type I CIITA but little type III or IV; CpG DNA induced their maturation and decreased types I and III CIITA, consistent with decreased MHC-II protein synthesis. CIITA types differ in regulation in various APCs under different stimuli, and the predominant type of CIITA varies at different stages of APC activation.