A new type of interference of cryoglobulins on hemocytometric tests is described. The precipitation of temperature-dependent proteins produced a pseudolymphocytosis on a three-part differential leukocyte count of Coulter S-Plus VI, whereas unaffected results, identical to the microscopical count, were obtained using the cytometer Coulter VCS. The laboratory detection of cryoglobulin interference on hematological data is very important in patients with underlying diseases, where the accuracy of absolute and differential leukocyte counts is critical for follow-up. Histograms from the Coulter S-Plus VI can help detect these cases.