De novo central nervous system (CNS) IgG measles antibody synthesis rate was determined in two patients with subacute sclerosing panencephalitis (SSPE). Cerebrospinal fluid and sera were incubated with different concentrations of measles virus to determine maximum absorption of IgG. The difference in IgG concentration before and after incubation was considered the amount of measles-specific IgG antibody. Various tests were done to assess the immunological specificity of the absorption test. Using our empirical formula, the de novo CNS IgG measles antibody synthesis rate for one patient with SSPE was 57 mg/day, or 59% of the total CNS IgG synthesis rate; for the other it was 145 mg/day, or 82%. Daily determination of CNS measles antibody by empirical formula provides a quantification of humoral immunity in SSPE that corrects for both passage of measles antibody across a damaged blood-brain barrier and the effect of an elevated blood antibody level.