Twenty-five red cell samples with a positive anti-IgG direct antiglobulin test QAT) were tested with human monoclonal IgM Jka and jkb antibodies. Red cell samples were first tested by a 5- minute incubation tube test with the monoclonal antibodies (MAbs). The same red cells were then chloroquine diphosphate (CDP)-treated, and retested. Eleven of the CDP-treated samples were also tested with conventional polyclonal antibodies (PAhs) that required a 37 degrees C incubation for 30 minutes, followed by an indirect antiglobulin test. The Jka and Jkb MAbs consistently gave the same phenotype results both on untreated DAT-positive red cells and on the same cells after CDP treatment. Two of the CDP- treated samples had diminished antigen expression with the MAbs, a finding that may have been caused by the CDP treatment. One untreated sample, which spontaneously agglutinated in a low- protein medium, was incorrectly phenotyped with the anti-Jka MAb, but both MAbs and PAbs gave the same correct results with the CDP-treated cells. These findings illustrate that the use of Jka and Jkb MAbs in phenotyping DAT-positive specimens is practical and beneficial.