The present invention is directed to antibodies having specificity for a heavy chain class at the same time as having specificity for a first light chain. Such antibodies can be used in a method of detecting or monitoring a malignant plasma cell disease comprising determining in a sample the ratio between the relative amounts of immunoglobulins having: (i) a heavy chain class bound to λ light chains; and (ii) immunoglobulins having the same heavy chain class but bound to κ light chains. More particularly, in one embodiment the ratio is determined after measuring the relative amounts of the respective immunoglobulins using: an antibody, or a fragment thereof, having specificity for a heavy chain class at the same time as having specificity for a first light chain in combination with either: (i) an antibody, or a fragment thereof, having specificity for the heavy chain class at the same time as having specificity for the second light chain; or (ii) an antibody, or fragment thereof, having specificity for the heavy chain and a further antibody, or fragment thereof, having specificity for the second light chain.