Although weanling mice are refractory to the tumor-inducing effect of polyoma virus infection, they are quite susceptible to infection per se, as evidenced by the uniform development of antibody after parenteral or intranasal inoculation of low doses of virus (1, 3). To characterize the pattern of infection in weanling mice, experiments comparable to those described were conducted. Swiss mice were infected when 3 to 4 weeks old by intraperitoneal injection of 0.2 ml. of undiluted tissue-culture fluid containing 10^6.5 mouse infectious doses of virus. Titers of selected organs were determined at various times after infection (table 8).

Table 8. Growth And Excretion Of Polyoma Virus After Intraperitoneal Inoculation Into Weanling Swiss Mice. Inoculum: 10^7.0 Mouse Id50 Virus Titer*

*Titers of tissue suspensions expressed per 0.2 ml. of 10 percent suspension; titers of urine expressed per 0.2 ml. of urine.

The growth pattern roughly paralleled that in newborn mice, but the maximal titers obtained were 3 to 4 logs lower, and antibody development appeared to be somewhat more prompt. The tissue extracts in table 8 were also tested for hemagglutinin, and all were negative at a 1:20 dilution. Despite the reduced titers of virus in the tissue suspensions, virus was occasionally present in urine and saliva, in the former occasionally in a relatively large amount. In 2 mice the titer of virus recovered from urine was 100-fold greater than from an equivalent mass of kidney. This could perhaps be due to the greater inhibiting effect of antibody in the tissue suspension or to propagation of virus in the lower urinary tract. As in the newborn mice, virus persisted at a low level in the tissues for prolonged periods, particularly in the salivary glands.