214 Arne Forssberg 



When DNA suspensions, isolated from Ehrlich ascites and 

 purified from proteins, were injected intraperitoneally in 

 quantities of about • 5 mg. in Ehrlich ascites of mice which 

 had been irradiated in the manner described, mitotic activity 

 reappeared in the tumour cell population somewhat earlier 

 and the number of cells in mitosis was higher than in samples 

 from animals treated by irradiation and saline injection only; 

 whether the DNA injection was given 2 hours before or 2 

 hours after irradiation, the effect was approximately the same. 

 In similar experiments with DNA isolated from calf thymus or 

 mouse liver no stimulating effect has so far been observed. 



Since the ascites fluid contains DNAse, injected DNA can 

 be expected to be enzymatically degraded at a rapid rate, 

 and a species specific character of the DNA should be lost. 

 Gale (1955) has shown that incorporation of amino acids in 

 Staphylococcus aureus is inhibited in cells disrupted with 

 ultrasonic treatment and deprived of their nucleic acids, but 

 that this faculty can be restored by adding either homologous 

 DNA or RNA. If the nucleic acids were enzymatically de- 

 graded, reactivation of amino acid incorporation took place 

 even after addition of heterologous nucleic acid degradation 

 products, e.g. from yeast. In view of these results, the mechan- 

 ism of the DNA effect in our experiments is so far obscure. 



One would expect the stimulation caused by Ehrlich DNA 

 to be rather transient as the depot is probably used up within 

 a short time, in contrast to what happens when X-ray-killed 

 cells are mixed with living cells. It can be assumed that the 

 lysis of dead cells is protracted over a fairly long period and the 

 material furnished — not only DNA but also other cellular con- 

 stituents — is available during correspondingly longer intervals 

 of time. 



It can be readily demonstrated by isotope-labelhng methods 

 that DNA from X-ray-killed cells, or at least important parts 

 of the DNA molecule, can be transferred to living cells of the 

 same sample during growth. We used cells which were labelled 

 in vivo with i*C-adenine, and harvested the cells some days 

 after injection when 97 per cent or more of the adenine had 



