Krugman and his co-workers laid the groundwork for active immunization against hepatitis B in 1970 to 1973. They discovered that a 1:10 dilution of hepatitis B infective serum lost its infectivity when boiled for one minute but retained its antigenicity and prevented hepatitis B in 70 percent of vaccinated subjects. Hilleman and his colleagues at the Merck Institute of Therapeutic Research developed a more sophisticated vaccine consisting of highly purified, formalin-inactivated HBsAg particles derived from the plasma of chronic carriers of the antigen. By 1978, data were sufficient to permit testing in a clinical trial.