Biologically active peptides that are derived from or are similar to sequences identical with the N-terminus of the .alpha.S1 fraction of milk casein. These peptides are capable of stimulating and enhancing immune response, protecting against viral infection, normalizing serum cholesterol levels, and stimulating hematopoiesis. The casein-derived peptides are non-toxic and can be used to treat and prevent immune pathologies, hypercholesterolemia, hematological disorders and viral-related diseases, alone or in combination with other peptides or blood cell stimulating factors.