AbstractLymphocystis, a virus caused, tumor producing disease of marine and fresh water fish was studied using various cytochemical methods for general proteins, histones, both classes of nucleic acids (DNA and RNA), and polysaccharides including mucopolysaccharides and acid mucopolysaccharides. Special interest is afforded the Feulgen‐positive cytoplasmic inclusions which accompany the virus infection of the cells, and concomitant increases in RNA and protein. Particular attention is given the chemistry and formation of the acid mucopolysaccharide capsule which develops around individual cells following viral infection. Possible significance of the capsule is discussed.