Electroimmunodiffusion on a cellulose acetate support medium was used to study the effect of heat treatment on the antigenic activity of bovine serum albumin suspended in water, simulated milk ultrafiltrate (J & K buffer), or saline (.85%) and of bovine serum albumin in milk per se. Heat treatments were at 70, 73, and 75 C for 0 to 30 min. The suspending medium influenced the loss of antigenicity of bovine serum albumin upon heat treatment. The loss in increasing order was J & K buffer, saline, and water. In milk, the loss of antigenicity of bovine serum albumin was 70 C for 30 min, none; 73 C for 30 min, 24.6%; and 75 C for 20 min, almost complete with no characteristic cone formation upon electroimmunodiffusion.