Double immunoenzyme labelling has progressed in the past few years to double labelling using monoclonal antibodies (MAB) raised in the same species. It presents the problem of cross-reaction between the primary antibodies and the enzyme-labelled secondary antibodies as well as that of excessively high background staining. These difficulties have been overcome in part by using normal mouse serum as a blocking agent. However, differentiation of the resulting colour reactions, and therefore interpretation of results, is difficult, especially when compounded by the presence of an intermediate colour due to the combination of two reactions. Because of this problem, normal mouse serum (NMS), BALB/c mouse serum and foetal calf serum (FCS) were tested for their effectiveness as blocking agents in a double-labelling procedure using two mouse anti-human MABs.