180 R. W. SCHLESINGER 



conditions are given whenever a saturating seed inoculum contains a pre- 

 ponderance of noninfectious virus. As with active or artificially inactivated 

 virus, effective blockade at the cellular level is not established until several 

 hours after exposure (Burnet et al., 1954). The existence of different kinds of 

 "incomplete" virus is borne out by the finding of a correlation between 

 interfering capacity and content of internal S antigen of different preparations 

 (Paucker and Henle, 1958). 



A true evaluation of the relative interfering capacity of incomplete virus is 

 complicated by the fact that all native preparations contain some infectious 

 virus as well (von Magnus, 1951b; Gard et al., 1952). Recourse can be taken to 

 systems in which even infectious influenza virus is capable of only limited 

 reproduction. This approach has revealed that incomplete virus is not as 

 potent an interfering agent as infectious virus. Thus, incomplete influenza 

 virus derived either from mouse brain or from undiluted passage series in 

 eggs gives no or negligible interference in mice against intracerebral doses of 

 WEE virus, while equivalent amounts (in terms of HA titer) of active 

 standard virus are highly effective (Schlesinger, 1951). More recently, Manire 

 (1957) found a similar discrepancy in the capacity of standard and incomplete 

 influenza virus to inhibit toxicity of intravenously administered homologous 

 or heterologous influenza virus. 



The findings of Henle and associates (Finter et al., 1955; Paucker and Henle, 

 1955a,b; Liu et al., 1956; Paucker and Henle, 1958) confirm substantial 

 functional similarities between spontaneously arising incomplete and artifici- 

 ally inactivated influenza virus. Axiomatically, different preparations of either 

 type are bound to be heterogeneous and to have variable populations. It is 

 doubtful whether interference experiments or the crude technique of undiluted 

 passage series would reveal subtle differences between incomplete and 

 inactivated particles. The genetic approach suggests the existence of such 

 differences. Burnet et al. (1954), in confirming the observation of Henle and 

 Liu (1951) that partially inactivated virus is capable of multiplicity reactiva- 

 tion, at the same time find that incomplete virus can contribute genetic 

 markers to progeny emerging from de-embryonated eggs doubly infected with 

 incomplete and active virus, and in this respect resembles heat-inactivated 

 virus (Burnet and Lind, 1954a). Gotlieb and Hirst (1956) find, in contrast, 

 that partially UV-inactivated virus can be reactivated by double infection 

 with active virus, while incomplete undiluted passage or mouse brain virus 

 cannot. Perhaps these conflicting findings are again related to the variations 

 of S antigen (Lief and Henle, 1956) and RNA (Ada and Perry, 1956) contained 

 in different preparations of incomplete virus. It is clear that much more work 

 has to be done along such lines before the interfering capacity of an incom- 

 plete particle can be clearly defined relative to that of an inactivated 

 particle. Perhaps greater emphasis on systems employing heterotypic 



