In cultured microspores from Brassica napus, embryogenesis can be synchronously and irreversibly induced by elevating the culture temperature to 32°C for a minimum of 8 h. Culture at 18°C allows gametophytic development to continue, and results in the formation of pollen in vitro. This allows us to study the temperature controlled switch in developmental fate from gametophytic development to embryogenic development by molecular means. Analysis of protein synthetic patterns by [35S]-methionine incorporation and 2-dimensional gel electrophoresis, revealed that 25 proteins were differentially synthesized during the induction of microspore embryogenesis. Most of these proteins (17) appeared to belong to the class of heat shock proteins (HSPs). Four of these HSPs have been identified by Western blotting using antibodies raised against HSP17, HSP68 and HSP70. One protein that was only synthesized under embryogenic culture conditions, and did not belong to the heat inducible HSPs, is a candidate marker for early embryogenic development.