In Bradysia hygida (Diptera, Sciaridae) well spread polytene chromosomes, free of cytoplasm, and with good morphology are consistently obtained if before squashing in acetic acid solution, the salivary glands are fixed in 7% perchloric acid containing small amounts of ferric ions (SAUAIA et al. 1971). We show here that, with regard to the preservation of total incorporated 3H-thymidine and with regard to the relative autoradiographic labelling of expanded chromosome regions as compared to the labelling of a non-expanded one, this method is equivalent to fixing the salivary glands in ethanol-acetic (3:1). We show also that if this kind of preparation is subject to mild acid hydrolysis a small amount of the total 3H-labelled material may be lost.
