Abstract Chromatin proteins of Balb/c-3T3 cell line and its temperature sensitive variants A83 and A8, arrested in G1 phase of the cell cycle at the restricted temperature, were analysed on SDS-poly-acrylamide gel by electrophoresis to detect major alterations that could be related to the G1 arrest. Appearance of two additional proteins (mol. wt. 80,000 and 20,000) and disappearance of another (mol.wt. 14,000) were thought to be significant. The protein of mol.wt. 20,000 was absent in the G1 −derivatives, suggesting a possible relationship to the appearance of the G1 phase.