The present invention provides a method of stimulating dendritic cells, comprising contacting the dendritic cells with methionine enkephalin. The present invention also provides methods of enhancing an immune response to an antigen in a mammal. In a first method, dendritic cells are isolated from the mammal. The dendritic cells are then contacted with methionine enkephalin for a time sufficient to stimulate the cells. Next, the stimulated dendritic cells are contacted with the antigen of interest for a time sufficient for the cells to process the antigen. The dendritic cells are then injected into the mammal. The dendritic cells may be injected into the animal either on their own, or along with methionine enkephalin and/or the antigen. In a second method, the antigen of interest and methionine enkephalin are administered to the mammal. The present invention also provides compositions for administration to a mammalian subject having a tumor.