An IgM 11-oxycorticosteroid-dependent antibody was identified in the serum of a patient with bladder malignancy, which at 4-37 degrees C reacted with all the 11-oxycorticosteroids tested but not with desoxycorticosterone, testosterone, nortestosterone and progesterone. The resulting drug-antidrug antibody complex combines nonspecifically with red blood cells causing agglutination. This reactivity is enhanced both by acid pH and by high drug solution concentrations as well as by ficin test. The antibody is complement-independent and has no blood group specificity. No in vivo or in vitro hemolysis was observed.