Described is an in vitro method for developing dendritic Langerhans type cells, for use in immunotherapy against cancer or infectious diseases and enhancement of graft survival in mammals. The method includes the steps of: culturing cells selected from peripheral blood monocytes and bone marrow cells in a medium containing platelets obtained from the same species or phylogenetically close species; and incubating the culture at 30 to 40.degree. C. for a period sufficient to enable formation of mature dendritic Langerhans type cells.