Complement-binding antibodies (2 examples of anti-Jka and 2 anti-kell) which inhibit agglutination of enzyme-treated cells have been investigated in the sera of 4 patients. The sera containing anti-Jka also contained rhesus antibodies (c + E and D + C) which could not be readily identified by an enzyme technique due to failure of the Jka+ cells of the corresponding Rh phenotype to react. The inhibition of the enzyme test reactivity was revealed using a control reagent added to each test after the initial examination for agglutination. Addition of ethylenediaminetetraacetic acid to the sera prior to testing resulted in identifiable reactivity with panel red cells. Failure to recognise this phenomenon in sera containing antibodies reacting only by an enzyme technique could result in misidentification, delay in providing compatible blood and transfusion of incompatible blood. Routine control of enzyme tests to detect false-negative results is recommended.