This study was conducted for morphological characterization, cloning efficiency, tumorigenecity and fusion capability of the S.U.99 plasmacytoma cell line. The morphological characterization revealed the presence of heterogenous populations of bipolar, small fibroblastoid cells, which contain single, large eccentric nuclei, some cells showed binucleated, multipolar large in size and the nuclei contain clumped chromatin. A single cell colony was isolated and studied. The cells were monitored for growth characteristics, morphological differences from the original cells. The growth curve of the cells was similar to most of the known continuous cell line and it has a doubling time of about 24 hrs. S.U.99 plasmacytoma cell line is tumorigenic and it produced tumor in the immunosuppressed mice. First group injected with 107 S.U.99 cells intraperitonally. Ascites tumor developed in 33.3% of the mice. Ascites smear, and histological sections revealed the appearance of plasmacytoma cells. A second group injected subcutaneously developed subcutaneous tumor in 20% of the mice. S.U.99 plasmacytoma cell line was found to be Hypoxanthine, Aminopterin, Thymidine (HAT) sensitive. Which were successfully fused with the splenocyte from an immunized animals via the use of the polyethylene glycol (PEG) as a fusogenic agent and about 92 % of the total wells were developed hybridoma colonies.