The secretion of milk proteins is one of the functions of the mammary gland during pregnancy and lactation and it may also occur to some extent during certain pathological processes. If it were possible to detect these proteins consistently and locate them accurately our knowledge of mammary gland physiology and pathology would be much improved. We have used an immunofluorescent method for this purpose and this preliminary communication records some of our results.
Milk was collected from lactating rats when the young were 10–15 days old, it was frozen immediately and stored at − 20°. When enough milk had been collected it was allowed to thaw, the fat was removed and casein was separated by high-speed centrifugation or by acid precipitation at about pH 4. Rabbits were immunized against this rat casein by a single s.c. injection of antigen plus Freund's adjuvant followed by booster i.m. injections of antigen alone,