The chromosomal progression during early in vivo passages was studied in 26 different Rous mouse sarcomas. Several sarcomas, originally having a normal, diploid stemline, showed steps in their heteroploid transformation in accordance with observations in operation samples of the primary tumours, i.e. (1) formation of variant cells, (2) development of a heteroploid sideline from them, (3) increasing superiority of the sideline leading to an S-shift and (4) numerical and/or structural changes not changing the heteroploid S-category. Among the hypodiploids there was a tendency towards decrease in chromosome number; the pseudodiploids revealed signs of a further reshuffling of the genic material; the trends among the hyperdiploids were either elevation of the S-number or appearance of markers; the polyploids showed the “Ising” phenomenon and increasing structural deviations. Polyploidization usually represented a fifth step for heteroploid sarcomas in the diploid region and the following changes for them were in accordance with the modificative step for the polyploids. Thus, the general progressional trend was a development of hypotetraploid-near-triploid stemlines; they showed many similarities to stemlines observed in advanced murine tumours. The present study revealed a 6th case with the chromosomal aberration double-minutes. The histopathological picture was a parameter showing correlation to the stemline category and often to major progressional changes. The progressional pattern found in Rous mouse sarcomas differed from that seen in murine polyoma tumours, mammary carcinomas and lymphomas-leukemias and also from the pattern observed during spontaneous in vitro transformation of normal mouse cells.