Chromosome studies of a solitary plasmocytoma in the femoral bone revealed a near-haploid chromosome number of 31-32 with a loss of one homolog of each chromosome pair except #1, #7, #9, #15, #19-21, and the sex chromosomes (XY). The cytogenetic findings have been compared with 16 cases of near-haploid neoplasms from the literature studied using banding techniques. A common feature present in 13 of the 16 cases reported was found to be disomy 21; the only chromosomes consistently present in one copy in all neoplasms were #2, #3, #4, and #5.