The cytogenetic analysis of transplantable in vivo melanotic and amelanotic lines of mouse B16 melanoma was performed. The second line arose by spontaneous alteration of the first one and these lines are the same in the rate of malignant growth during passages in vivo. The melanotic line shows stable near-diploid karyotype with modal chromosome number 41. The amelanotic line is mostly hypotetraploid karyotype with modal chromosome number 76. The balance of the diploids in the melanotic line is disturbed by the fact that chromosome 15 is partially trisomic, chromosome 6 trisomic, and chromosome 13 and X monosomic. Four marker chromosomes were common to both lines: M1-rob(12;12), M2-rob(5;15), M3-minute, M4-inv(1), M5-del-(14q-), but in the amelanotic line in most cases their number was doubled. Additionally, in the amelanotic line other specific aberrations rob(6;6) and rob(16;16) were observed. Both lines had comparable SCE frequencies. Constitutive heterochromatin in amelanotic line revealed the nonrandom deletion of the heterochromatic segments in chromosome 11 and, occasionally in chromosome 19.