Abstract Introduction/Objective In rare cases, the conventional immunofixation gel electrophoresis technique fails to detect the light chain of an M-protein. We report a case of immunoglobulin (Ig) D multiple myeloma with a hidden lambda (λ) light chain. Methods/Case Report Capillary electrophoresis (CE) (Sebia CAPILLARYS 2) was used to detect and quantify M- proteins in serum specimens. Immunosubtraction (IS) on the CAPILLARYS 2 systems was used to identify the classes of M-proteins. Conventional gel immunofixation electrophoresis (IFE) was performed, using monospecific antisera for IgD, IgE, kappa (κ) or λ in the Sebia HYDRASYS system, and IgG, IgA, IgM, κ or λ in the Helena SPIFE3000 system. Beta-mercaptoethanol (BME) with Fluidil were used as reduction agents. Results (if a Case Study enter NA) Results of serum CE showed two abnormal peaks in beta 2 and gamma regions, suspected to be positive for M-proteins. IS results showed subtraction for λ light chain only in both peaks, suggesting two monoclonal λ light chains. In contrary, no monoclonal λ light chain was detected in gamma region by IFE (Sebia). Epitope masking in the folded monoclonal protein was suspected to cause the “hidden λ light chain” and was further investigated by two laboratory approaches. IFE performed on the Helena SPIFE3000 system found two λ bands in beta 2 and gamma regions, which was consistent with the results from IS. The treatment of BME with Fluidil helped unmasking the hidden epitope and revealed the λ band in gamma region on IFE (Sebia). Conclusion The medical laboratories should be aware of the described scenario. The failure to detect light chains of certain intact M-proteins is most likely due to the structurally inaccessibility of light chains. It is recommended that treatment with reduction agents or use of an alternative methodology or IS might be helpful for investigating suspected heavy chain only cases, due to the limitation of conventional methodology.