Recently, the author found a typical case of IgM-κ type M protein interfering with the detection of CRP in whole blood samples, and evaluated the anti-interference ability of several CRP detection systems. Reviewing the biochem. results of this patient, total protein was high (86.7 g/L), albumin was low (33.8 g/L), and the albumin/globulin ratio was inverted (0.6). However, liver enzymes were normal, suggesting that the inversion of albumin/globulin ratio was not caused by liver dysfunction. The Ig IgM level was detected, and the result was severely over-limited, about 45 g/L. Finally, the results of serum protein electrophoresis confirmed the presence of M protein in the patient, and further typing suggested that it was IgM-κ type M protein.