404 S. GARD AND O. MAAL0E 



proceeding as a first-order reaction, as are the reactions in tlie nucleic acid 

 responsible for the loss of reproductive capacity; (b) the "affinity" or access- 

 ibility of the nucleic acid to formaldehyde is at any time proportional to the 

 remaining "free" fraction of the protein surface. The result is a probability 

 function of a Poisson expression, of which the empirical equation (1) offers a 

 very close approximation (Gard, 1957). The isotope studies of Meriwether 

 and Kosenblum (cf. p. 398) seem to point in the same direction. These authors 

 describe three stages in the incorporation of formaldehyde: (a) during the 

 first 15 hours the surface reactions seemed to predominate; (b) next, followed 

 3-4 days of an approxunately semilogarithmically decreasmg rate of incor- 

 poration; (c) later, the process was further slowed down and had not come 

 to a standstill even after 36 days, when the experiments were interrupted. 

 Probably the subdivision in stages (b) and (c) is artificial. The most likely 

 explanation is that, as in inactivation studies, the deviation from a semi- 

 logarithmic course becomes significant only when a sufficiently wide reaction 

 range is covered. 



It was already pointed out, as a corollary of the "membrane hypothesis," 

 that the ratio of inactivation rate/formaldehyde-concentration increments 

 should be expected to be less than unity in the range of low concentrations 

 of the chemical agent. With TMV, Cartwright et at. (1956) observed values of 

 roughly unity; the four points on which this assumption is based are too 

 scattered, however, to permit any definite conclusions. Lauffer and Wheatley 

 (1949), studying influenza A virus, recorded a value of about 0.8. As yet un- 

 published Swedish observations on poliovrrus and bacteriophage indicate 

 that inactivation in formaldehyde concentrations below 0.002 M is very 

 slow, the increment ratio approaching unity only from concentrations of 

 0.004 M and upward. 



As another corollary, pretreatment with formaldehyde should be expected 

 to render the survivhig virus more resistant to inactivation by most other 

 chemical agents as well. In spite of the fact that combined chemical treatment 

 — formaldehyde followed by ^-propiolactone (Lepme, 1957) — is bemg 

 applied in commercial production of poliovirus vaccine, no kinetic studies of 

 this problem have been published. 



The question of avidity changes in the course of treatment has only re- 

 cently become a matter of interest. It has been a general experience in large 

 scale production and testing of poliovirus vaccine that formaldehyde-treated 

 virus may produce cytopathogenic changes m tissue cultures as late as 3 to 

 4 weeks after inoculation, which never happens with untreated virus. As 

 already mentioned, this phenomenon seems to be at least partly explained 

 by a reduction in the adsorption rate, most probably an effect of charge and 

 solubihty changes in the virus protein. Similar observations concerning other 

 viruses have not yet been reported. The immediate practical consequences of 



