Two human malignant mesotheliomas were studied by banding techniques. Both neoplasms had a hypodiploid stemline. Five and 8 marker chromosomes, respectively, were included in the stemline karyotypes. Monosomy 14, monosomy 22 and a no. 13 with an interstitial long-arm deletion were deviations found in both neoplasms. Closely located regions in no. 1 and no. 3 were involved in both tumors in the formation of other marker types with dissimilar morphology.