Autovaccines are therapeutic vaccines manufactured from a disease causing micro-organism for individual treatment of patients, animals, or sometimes herds to treat chronic or recurrent infections. Despite the common use of autovaccines in veterinary medicine, their mechanism of action, i.e. the immunologic effector mechanism activated after administration, has never been investigated. Here we present data concerning the use of autovaccines to treat metritis infection in a group of dairy cows. Following autovaccination we observed a significant decrease in CD4+ cells paralleled by an increase in T-cells expressing the gammadelta-T-cell receptor (gammadelta-TCR) in the peripheral blood of the treated animals. Lymphocyte proliferation assays showed an initial increase in antigen-specific responsiveness followed by a decrease in this responsiveness during autovaccination treatment. We therefore conclude that administration of an autovaccine leads to the activation of immunologic effector mechanisms which contribute to recovery of the diseased animals.