SummaryTwo kinds of inclusion bodies, “A” and “B” type, exist in tissues infected with ectromelia, fowl pox and vaccinia viruses. “B” type inclusion bodies of these 3 pox group viruses have almost the same morphological and cytochemical characters. The “A” type inclusion bodies of these viruses differ according to virus and host cell. The frequency of the appearance of the “A” type inclusion body of vaccinia virus is lowest as compared with the “A” type inclusion body of the other 2 viruses. The so-called Guarnieri body in smear preparations stained by Giemsa solution is “B” type inclusion body. It differs from the so-called Guarnieri body in sections stained by hematoxylin-eosin which was named “A” type inclusion body.