In this article, state of art and perspectives in mammary gland biotechnology are reviewed. Recent progress in recombinant DNA technology as well in embryo manipulation and transfer has made the introduction of specific genes into the germline of animals relatively easy. With appropriate genetic constructs, the expression of the inserted genes in transgenic animals can be controlled in a tissue-specific and in a differentiation-specific manner. Thus, it is now possible to consider alteration of the composition of milk produced by lactating animals in a variety of ways. There is a growing list of foreign milk proteins that have been expressed, and one can envisage placing almost any protein gene of interest under the control of promoter of a milk protein gene. Many human proteins of a potential pharmaceutical use may be now produced in the mammary glands of laboratory or farm animals. Modification of milk composition can be extended not only to produce proteins of commercial value but also, by manipulation of key metabolic enzymes, to fat, lactose, and other components of milk. Many alternations in ruminants' milk composition, including 'humanization' of cow's milk, are planned, however, these manipulations must avait the development of totipotent embryonic cell lines (ESC) of farm animals, cells that enable gene manipulation by homologous recombination. In spite of a great progress, many obstacles and difficulties still exist on the way to economical production of human paharmaceuticals in farm 'transgenic bioreactoractors'. These dificulties are discussed in detail.