Cultures of mink testis and mink kidney cells were inoculated with 10% extracts and filtrates of tissue from plasmacytosis-affected mink. Specific morphological changes were observed in cultures of kidney and testis cells. Millipore filtration experiments suggested the size range of the agent to be from 10 to 50 mmu. Inoculation of fluids from the 2nd, 4th and 8th tissue culture passages of the agent induced plasmacytosis in adult mink.