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The reaction was studied of the J5-monoclonal antibody with the common acute leukemia antigen (CALLA) on non-malignant human tonsil lymphatic cells, which were double labelled also with antibodies against the B1 and HLA-DR antigens or the kappa and lambda light chains. There were few cells (mean 0-1.4%) which were positive with J5 but not B1. Other percentages of labelled cells depended on the order in which the antibodies were added. If the J5-antibody was added first, more B1 positive cells are found than if the B1 antibody is added first. Similarly, a significant (p less than 0.01-p less than 0.001) increase (3.3-18.8%) in the percentage of cells positive with J5 was found when the B1, kappa, and HLA-DR antibodies were added first, but not if antibody against lambda chains is added first. This difference was most pronounced and most significant for B1. This difference can be explained inter alia by steric differences between antibody conjugated and non-conjugated antigens followed by unmasking of other antigens. Double labelling is thus not equal to two single labellings.