A new human myeloma cell line, KMS-18, was established from a 58-year-old male with multiple myeloma associated with hyperammonemia. The original leukemic cells and established KMS-18 cells possessed several of the same chromosomal abnormalities, including add(1)(q32), add(10) (q24) and add(17)(p11). In addition, the KMS-18 cells showed novel t(4;14)(p16.3;q32.3) masked translocation which was determined by the FISH method. Moreover, we compared the ammonia production in culture medium of the KMS-18 cell line with that of non-myeloma hematological malignant cell lines and a hepatocellular carcinoma cell line. KMS-18 produced higher levels of ammonia in medium than the other cell lines examined. This new cell line may prove helpful in analyzing the role and biological mechanisms of the t(4;14)(p16.3;q32.3) translocation in myeloma and also in investigating hyperammonemia in cases with myeloma.