Brain tissue and serum from mice intracerebrally injected with 1 microgram of staphylococcal alpha-toxin contained elevated amounts of a naturally occurring brain tissue component(s) called muscle-relaxing factor (MRF). MRF induced reversible, generalized, flaccid paralysis of mice after intracerebral but not intraperitoneal or intravenous administration. MRF (i) was soluble in Hanks balanced salt solution and in acidified (pH 2) Hanks balanced salt solution, in which it partitions into ethyl acetate, acetone, and methanol; (ii) was separated from some pigments by thin-layer chromatography on silica gel plates; (iii) did not comigrate with prostaglandin and leukotriene standards during high-pressure liquid chromatography with a mu Bondapak fatty acid column; and (iv) did not contain amino acids, exhibit absorption maxima at a wavelength range of 210 to 600 nm, or fluoresce when exposed to UV light. MRF has been detected in rabbit brain that has been stored frozen at -70 degrees C and has been enhanced in vitro in slices of both mouse and rabbit brain following incubation of the brain slices with staphylococcal alpha-toxin. Studies to identify the chemical nature of MRF and the mechanism by which, in mice, it induces reversible, flaccid paralysis of voluntary muscle are continuing.