Production of human procollagen or collagen in cells which ordinarily do not produce these molecules is effected by constructing expression systems compatible with mammary glands of non-human mamals. For example, expression systems can be microinjected into fertilized oocytes and reimplanted in foster mothers and carried to term in order to obtain transgenic non-human mammals capable of producing milk containing recombinant human procollagen or collagen. Human procollagen or collagen produced in this manner can be made of a single collagen type uncontaminated by other human or non-human collagens.