The effects of an ethanol-acetone extract from soy flour on serum lipids and thyroid hormones were studied in rats and hamsters. In Study 1, male Sprague-Dawley rats were fed diets containing protein from ethanol-acetone extracted isolated soy protein (ISP-), nonextracted ISP (ISP), casein (casein-), or casein to which 0.36 mg ethanol-acetone extract/g protein was added (casein+). Rats fed either ISP-containing diet had lower serum total cholesterol concentrations compared with those fed either casein diet (P < 0.05). Lower serum LDL-cholesterol concentrations were present in rats fed either ISP-containing diet and in those fed casein+ compared with those fed casein- (P < 0.05). Adding the extract to casein (casein+) produced higher serum thyroxine concentration and free thyroxine indices compared with all other groups (P < 0.05). In Study 2, male Golden Syrian hamsters were fed experimental diets containing protein from ISP, ISP with added ethanol-acetone extract (0.36 mg extract/g protein; ISP+), casein-, casein+ (0.36 mg extract/g protein), or casein with twice the level of extract (0.72 mg/g protein; casein++). Lower serum total cholesterol and LDL (LDL + VLDL + IDL) cholesterol concentrations were observed in hamsters fed ISP, ISP+ or casein+ compared with those fed casein- (P < 0.05). Addition of the extract at higher levels to casein (casein++) did not lower serum lipids relative to those fed casein-. Serum thyroxine concentration and the free thyroxine index were greater in both ISP groups as well as in hamsters fed casein++ compared with those fed casein- or casein+ (P < 0.05). In conclusion, both studies show that protein from soybeans decreases serum total and LDL cholesterol while the effects on thyroxine are different in rats and hamsters. It is also apparent that the ethanol-acetone extract of soy flour produces changes in serum cholesterol, particularly in the LDL fraction in both species.