A method of producing a genetically transformed fluid-producing plant comprises:-
i) inserting into the plant tissue a gene or gene fragment controlling the expression of a target product, and ii) regenerating a plant from said tissue, the genetically transformed plant being capable of expressing the target product in the fluid that it produces. There are also provided clones of genetically transformed fluid-producing plants which contain in their cells a chromosomal insert such that the target product is expressed in the fluids that the plants produce. There is further described a method of producing a protein or other target product which comprises:-
i) harvesting the fluid from a genetically transformed fluid-producing tree or plant, or a clone thereof, and ii) recovering the target protein or other product from said fluid. Most preferably, the plants are rubber ( Hevea ) plants and the genes are foreign genes that code for pharmaceutically valuable protein products which can be harvested in the latex produced by the plants.