The combination of As and IFN induced morphologic changes characteristic of apoptosis when stained with Hoescht 33342. The nuclei of HUT-102 cells treated with the combination of As and IFN (C) underwent apoptosis, as shown by their characteristically shrunken and intensely-fluorescent nuclei (arrowheads). Control cells (A) displayed the typical morphology of nuclei, as did HUT-102 cells treated with As only (data not shown). Under light microscopy, cells treated with the combination of As and IFN (D) were less numerous and exhibited membrane blebbing when compared with control cells (B).