A novel immunization procedure for eliciting monoclonal antibodies ( McAbs ) is described. With intrasplenic inoculation only small amounts of immunogen are required. As little as 20 micrograms of protein antigen or 2.5 X 10(5) cells have been found sufficient to immunize mice or rat spleen cells for the production of specific McAbs . A high proportion of hybridomas secreting McAbs against cell surface antigens and soluble proteins has been obtained with this immunization procedure. The system could facilitate McAb production in many instances in which only small quantities of immunogen are available.