We report a case of multiple myeloma showing marked differences in serum Immunoglobulin G (IgG) levels between serum protein electrophoresis and turbidimetry. A 47-yr old man was admitted to our hospital due to severe back pain and diagnosed as having IgG-kappa type multiple myeloma. Serum protein level was 14.4 g/dL at the time of diagnosis. Serum IgG level was 8.5 g/dL by serum protein electrophoresis, but 11.6 g/dL by turbidimetry. The patient's clinical conditions had improved after receiving VAD (vincristine, adriamycin, dexamethasone) and VTD (vincristine, thalidomide, dexamethasone) chemotherapy and there were no differences in IgG levels between electrophoresis and turbidimetry when serum IgG levels were less than 3.0 g/dL. According to this, we considered that both protein electrophoresis and turbidimetry should be needed to quantify serum immunoglobulins for diagnosis and follow-up of the patients with monoclonal gammopathy.