A procedure for in situ liquid propane jet-freezing of unfixed and unglycerinated monolayer cell cultures is described. We have used the method to study monolayer cultures of rat Sertoli cells and human monocytes. The described procedure gives a cooling rate during specimen freezing high enough to yield adequately frozen specimens even in the absence of any cryoprotectant. Use of the procedure therefore eliminates artefacts usually caused by cell fixation and addition of cryoprotectives.