Transverse cryosections, 6-8 mum thick, were cut from unfixed biventer cervicis muscles of chicks and quadriceps muscles of humans, mounted on glass slides and incubated for 1h in either isotonic phosphate buffered saline, pH 7.3 (PBS), or crude venom of venom of Pseudechis colletti at concentrations between 2.1 and 210 microgml(-1) in PBS. They were then exposed to a fluoresceine-conjugated alpha-bungarotoxin to label ACh receptor sites. Exposure to the crude venom of P. colletti prevented the labelling of acetylcholine (ACh) receptors in chick muscle in a dose-dependant manner; at a concentration of 2.1 microgml(-1) labelling fell by 20% and at a concentration of 21 microgml(-1) by more than 90%. In contrast, exposure to the venom at concentrations as high as 210 microgml(-1) had no effect on receptor labelling in human skeletal muscle. The results suggest that ACh receptors in human skeletal muscle are relatively resistant to the postsynaptically active neurotoxins in the venom of P. colletti. The data explain the apparent anomaly that the venom blocks neuromuscular transmission in isolated nerve-muscle preparations of the chick whilst human subjects of envenoming bites by P. colletti exhibit no overt signs of neurotoxicity.