Cell density dependent DNA replication in Ehrlich ascites tumour cells.

DNA replication of Ehrlich ascites tumour cells was investigated in suspensions with different cell densities by incorporation in vitro of tritiated thymidine and alkaline sucrose gradient analysis of the newly formed DNA. It is demonstrated that the incorporation of [3H]thymidine and chain growth of newly made DNA decreases with increasing cell density. The inhibition of DNA synthesis observed at high cell densities can be prevented if diffusible substances are removed by incubating the cells in dialysis tubes. This indicates that the changes in DNA synthesis are caused by diffusible inhibitors released from the tumour cells.