The prevalence of antibody to human immunodeficiency virus (HIV) was determined in various commercial substrate plasmas used in clotting factor assays, and viral isolation was attempted from both seropositive and seronegative samples. Antibody to HIV was detected in 13 of 13 plasma substrates used for Factor VIII assays and in 2 of 3 plasma substrates used for Factor IX assays. Antibodies were not detected in any of the other factor-specific substrates. Virus could not be isolated from any of the seropositive samples after 28 days in culture. Heat treatment of the samples under conditions known to inactivate HIV in plasma products indicated that heating the lyophilized substrate plasmas at 60 degrees C for up to eight hours had little effect on factor substrates and factor assays. Progressive loss of Factor V in the deficient plasmas was the most serious effect produced by heat treatment.