The availability of biopharmaceuticals has been increasing over the past decade and as their patents expire, the emergence of biosimilar agents approaches. The primary issue of concern for the safety of these agents is the potential for immunogenicity. Both product- and host-related factors have documented impact on the immune response, but many factors are still unknown. Although in many cases the presence of antibodies may have little clinical consequence, the upsurge of pure red cell aplasia cases further increased concerns about potential clinical consequences of extensive use of biopharmaceuticals and biosimilars. Available laboratory measurement methods are insufficient to predict biological and clinical properties of biopharmaceuticals, or even to compare their bioequivalence. Comparison of results from different studies is complicated by the variability of assay measurements, presentation of data and lack of standardization.