Characteristics of the Mammary Tumor Milk Agent in Serial Dilution and Blood Studies. ∗

The mammary tumor milk agent has been shown to be essential for the regular development of mammary cancer in mice. 1 It is usually transferred by nursing 1 and is present in various tissues^3-6 as well as the milk. The infectious agent has the characteristics of a virus. 7

This report gives data on the activity of the milk agent for mammary cancer in blood plasma, blood-cell suspensions, and serial dilution of extracts made from mammary tissue.

Blood was obtained from the hearts of mice known to harbor the agent. Either females of the A stock, 75 to 105 days of age, or females (A and C3H stocks and their hybrids) that had developed spontaneous mammary cancer were used. The blood was citrated to prevent coagulation but moderate hemolysis occurred. Separation of the cells and the plasma was by centrifugation (approximately 4,000 r.p.m. for 10 minutes) and the cells were washed with saline several times before they were used. The cells were suspended in saline and the plasma was injected either undiluted or diluted in saline (see table for details).

As test animals mice were used that were susceptible for mammary cancer and lacking the milk agent. They were injected intra-peritoneally when from 3 to 5 weeks of age. The incidence in control mice maintained as breeding females was approximately 1%.

The results for the blood studies are presented in Table I. Many of the mice in one series are living at 15 months of age.

The donors of the lactating mammary tissue were AZF₁ females that had had from 5 to 7 litters. The glands were macerated by means of a press, and serial dilutions were made with triple distilled water. Each animal received 1 cc of the suspension intraperitoneally and the surviving animals are 15 months of age. The preliminary results are tabulated in Table II.