The proposed continuation project will further the study of three areas of research which pertain to the sex differential and age-related immune depression in BALB/c mice. First, it will be determined if sex hormones can alter age-related immune depression between one and two years of age. Second, it is important to measure spontaneously levels of immunosuppressive factors, other than prostaglandin E, such as immune complexes, corticosteroid hormones, etc. A third area of research will be the continuation of the study on animals not exposed to the stress of infection at early ages. Female, as well as male mice, cesearian-derived and raised pathogen-free, will be compared to those conventionally raised to see whether their mitogen and alloantigen responses are comparable, at specific intervals in the aging process.