The acceptability of viral vaccines manufactured in culture of continuous cell lines opened the way to a new technology of vaccine preparation. The large scale cultivation of continuous cell lines contributed greatly to the improvement of the safety and the consistency of viral vaccines. Experimental studies concerning residual cellular DNA and its oncogenic potential showed clearly that this material is not able to induce tumours. The small amounts of cellular DNA present in the final product, and the deleterious effect of the vaccine manufacturing process on the integrity and biological activity of DNA, both contribute to regarding residual cellular DNA as an issue of no practical importance. There is no evidence that limits on the concentration of residual cellular DNA in the final product is scientifically justified.