Immune serum globulins (ISG) can be made substantially free of infectious retroviruses by storing the ISG in a liquid state at conditions of pH, temperature and time sufficient to inactivate substantially all infectious retroviruses. Preferred inactivation methods involve use of either of two specified storage conditions: (1) at a pH equal to or less than about 4.25 at a temperature of about 27.degree. C. for at least 3 days, or (2) at a pH equal to or less than about 6.8 at a temperature of about 45.degree. C. for at least about 8 hours.