The invention is based on the finding that microparticles can be produced by conditionally-immortalised cells. The conditionally-immortalised cells may be stem cells. The Examples show the successful harvest of microparticles from conditionally immortalised neural stem cells and CD34+ cells. Conditional immortalisation provides a constant supply of clonal cells that produce microparticles such as exosomes. The conditionally immortalised cells are useful as “producer cells” for microparticles such as exosomes, which are typically harvested or isolated from the conditionally-immortalised cells.
