Neutralizing antibody activity in cervical mucus to type 1 herpes virus was detected in 24 of 28 patients, and to type 2 herpes simplex virus in 18 of 24 patients. The neutralizing antibody activity resisted heat inactivation for 30 minutes at 56 degrees C, was independent of complement and followed first order kinetics. There was evidence of antibody against both virus types in immunoglobulin fractions IgG and IgA, the latter containing approximately threefold greater neutralizing antibody activity per unit of immunoglobulin concentration. Type 1 and type 2 neutralizing antibody activity showed a positive but weak correlation and type 2 neutralizing antibody activity showed a positive but weak correlation and a type-common immunoprecipitin was identified in all concentrated pooled mucus samples. However, type-specific neutralizing antibody against both virus types was identified in pooled mucus samples by heterologous absorption techniques. There was a relatively higher average type 2 neutralizing antibody activity in the mucus than in the serum and there was no correlation between serum and mucus antibody levels for either virus type. These observations support the concept of an independent local antibody system for herpes simplex virus in the uterine cervix.