The decreased vitality of ram and bull spermatozoa caused by sudden cooling (cold shock) may be largely prevented by the presence of egg yolk in the dlluent.The activity of egg yolk in preventing cold shock lies in the alcohol-soluble, acetone-insoluble fraction. The phospholipid lecithin may be isolated from this and will prevent cold shock in concentrations as low as 0·12 per cent.The protective action of lecithin for ram spermatozoa is greater at pH 6·5 than at a neutral or alkaline pH.1. INTRODUCTION The collection and storage of semen subjects spermatozoa to numerous environmental changes, which all tend to reduce the number of living and active cells. Sudden changes in temperatwe affect the vitality of spermatozoa; Milovanov (1934) calls the harmful effect of sudden cooling, "temperature or cold shock." Thus, Birillo and Puhaljskii (1936) observed that a fall in temperature from 40 to O°C in 30-60 min caused temperature shock to ram and bull spermatozoa. Cooling over 2 hr caused less shock and if it was prolonged to 3 or 4 hr, cold shock did not occur. However, positive results with ram semen were only obtained with artificially buffered specimens. Gladcinova (1937) reported that rapid cooling of horse, bull, and ram semen from body temperature to 5 or lOoC, caused reduced activity which could not be restored by rewarming. Cooling to 20 or 30°C did little harm. It was also noted that temperatureshocked spermatozoa rapidly died at 37°C.Ram semen was investigated by Gunn, Saunders, and Granger (1942), who found no advantage in slow cooling, but placed the semen in a container at 4°C as soon as collected. A cooling rate of not more than 5°C an hour was recommended by Easley, Mayer, and Bogart (1942) for bull semen, but semen diluted with egg yolk could be cooled rapidly, although sudden cooling to O°C caused significant motility loss. Earlier, Phillips and Lardy (1940) had recommended the use of egg yolk as a protective agent against cold shock and for the storage and transport of semen.Epididymal spermatozoa of the bull and boar have been shown to be very resistant to temperature shock, but after ejaculation the cells have little resistance (Lasley and Bogart 1944;Lasley and Mayer 1944). The addition of egg yolk to the semen before cooling restores resistance.