Efficacy of a therapeutic to enhance antitumor immunity in a patient is predicted, where the therapeutic is one that targets an immunomodulatory leukocyte membrane protein (ILMP) to enhance immune activity. Peripheral blood sample from the patient is tested for levels of monocytes having specific cell surface markers (CD14+, HLA-DRlow) prior to treatment. Low levels of monocytes of this type (PBM14+HLA-DRlow) indicate a greater likelihood of therapeutic efficacy. In specific exemplary embodiments of the invention, the therapeutic is an antibody to CTLA4, such as ipilimumab or tremelimumab.