Titration of the milk agent virus in milk and lactating mammary gland cells.

their subsequent development of mammary cancer (1, 4). Although data from previous experiments would suggest that this virus multiplies rather rap idly during the growth of mammary cancer through successive transplants (3), the present study was undertaken in an attempt to obtain data concerning the rate of duplication of the virus during the period of lactation. Observations in this laboratory indicate that during mid-lactation the mammae of mice produce about twice their own weight in milk each day. Such an outpouring of fluid containing a high concentration of virus would soon lead to a great reduction of virus in the gland tissue, unless viral duplication was proceed ing at a rather rapid rate. If it were found, there fore, that the concentration of the virus remained high in the gland during lactation and yet was es sentially as high in the milk as in the glandular tissue, it would seem to be evidence indicative of a rapid multiplication of the virus during lactation. In the course of such an experiment, data as to the relative merits of milk and lactating breast tissue as source materials for the purification of the virus would also be obtained.