AbstractBalb/c mouse embryo cultures that are carried under a rigid transfer schedule that minimizes cell‐cell contact evolve into permanent lines that possess properties very similar to 3T3. From the same embryo cultures, a transfer schedule where cell‐cell contact is extensive leads to lines (Balb/3T12) that form multiple cell layers and have saturation densities up to 25‐fold higher than the Balb/3T3 lines. All the lines are hypotetraploid.Comparison of the Balb/3T3 lines with 3T3 reveals similar morphology, ability to grow at high dilution, low saturation density, and high susceptibility to transformation by SV40 virus. The Balb/3T12 lines, which are continually maintained at high cell density, show little improvement in cloning efficiency. Infection with SV40 produces transformants that can be selected by their ability to form colonies at low cell densities. Using the appropriate selective system for each line, it appears that SV40 T antigen induction and transformation in Balb/3T3 and Balb/3T12 are compa
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