The bulky production of recombinant proteins can be achieved by procaryotes or eucaryotes cells. Cells from higher eucaryotes may be required when proteins have to be modified post-transcriptionally (glycosylation phosphorylation, cleavage, folding...). Cells from higher vertebrates in culture are used to prepare proteins like human factor VIII and erythropoietin. The use of transgenic organism has been suggested to reach the same goal. Indeed a whole living organism allows a very potent amplification, the number of cells involved in the biosynthesis of the recombinant proteins being very numerous and in the best metabolic conditions. Biological fluids (blood, milk, insect hemolymph, egg white...) and possibly organs from transgenic animals are a priori the best sources of recombinant proteins. Blood is abundant and it is a by-product of slaughter house. Its composition is relatively complex and the circulating recombinant proteins may heavily alter health of animals. Milk is very abundant, its composition is relatively simple, it is poor in proteolytic enzymes and it can be collected easily. Hemolymph from insects is relatively scarce. Egg white will be a possible source of recombinant proteins, when transgenesis has become more accessible in birds. Organs from transgenic animals should be solicited only when a particular cell type is required for the biosynthesis of the recombinant proteins. Milk appears therefore, presently, as the best source of recombinant proteins from transgenic animals. About 15 public and private laboratories try to use these techniques. They consist in preparing vectors containing regulatory regions of one of the milk proteins genes and the coding part (cDNA or gene) of the corresponding proteins to be produced. The transfer of these gene constructs to mouse, rabbit, sheep, goat, pig, shows that these techniques are indeed very promising. A single protein, human alpha 1-antitrypsin produced in milk of transgenic sheep, has presently reached the preparation at an industrial scale. This method has two theoretical limitations: 1) some of the proteins secreted in milk may be not matured as their native counterparts. Experiments carried out so far (about 20 proteins has been produced at an experimental scale) indicate that the mammary cell is able to achieve glycosylation in a correct way; 2) a significant proportion of the recombinant proteins migrate from the alveolar compartment of the mammary gland to blood circulation and they can alter health of lactating animals.(ABSTRACT TRUNCATED AT 400 WORDS)