Washed cells of Mycoplasma pneumoniae mixed with complete Freund's adjuvant were capable of producing lethal anaphylactic shock in mice when injected repeatedly into the foot pad. The causative agent is associated with horse serum contained in the culture medium but cannot be removed from mycoplasmal cells by repeated washing with phosphate buffered saline. Mycoplasmas grown in medium containing rabbit serum had a similar effect. No lethal anaphylaxis occurs when M. pneumoniae is injected without adjuvant.