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Whe Dejah Thosis is critically injured, John Carter goes in search of the scientist Ras Thavas, whose experiments to create synthetic men are not going well. When one of the vats explodes, a mass of living tissue is released which threatens the planet.

"Synthetic Men of Mars" was first published in Argosy Weekly (January 7, 14, 21, 28 February 4, 11 1939). The first hardcover edition was published by ERB, Inc Tarzana, March 15, 1940. The dust jacket and five interiors were done by John Coleman Burroughs.