Multiple myeloma (MM), like other hematological malignancies, has both normal and clonal neoplastic cells coresiding in the bone marrow. To perform interphase fluorescent in situ hybridization (FISH) analysis accurately, for the detection of clonal chromosome abnormalities, it is crucial to restrict the analysis to the tumoral cells. The correct detection of these abnormalities may have diagnostic, prognostic, and therapeutic implications. Our laboratory has developed a clone-specific cytoplasmic immunoglobulin (cIg) staining method coupled with FISH (cIg-FISH) for the study of plasma cell (PC) neoplasms. An unlimited combination of DNA probes can be used with this technique to examine the genetic changes that frequently occur in patients with these diseases, one of which is abnormalities of chromosome 13. Using the two techniques simultaneously (i.e., cIg-FISH), we have demonstrated that approx 50% of patients with MM have abnormalities of chromosome 13, mostly monosomy, and when present involving the majority of the cells. In this chapter, we present the technical methodology for detecting chromosome abnormalities and how to restrict the analysis to the clonal PCs.