A negative isolation method for separately isolating preparations of Th1 and Th2 helper lymphocytes from peripheral blood mononuclear cells involving the use of novel combinations of monoclonal antibodies to separately sequester specific Th1 and Th2 lymphocytes and contaminating leukocytes and erythrocytes, adding a magnetic colloid to the cells, and using a magnetic column for fractionation of Th1 and Th2 cells. Imbalances in the relative numbers of Th1 and Th2 lymphocytes can be used in the diagnosis and prognosis of human diseases.