The effect of an efficient bactericidal solution on the hatching of Haemonchus eggs was studied and it was found that a large proportion remained viable. Based on former work, a medium was devised in which the bacteria-free eggs hatched and the larvae developed to the infective stage. These larvae were morphologically and otherwise normal, except that they were slightly smaller than faecal-grown specimens, although the size ranges overlapped. Infection tests proved that Haemonchus larvae grown under conditions of sterility were normally infective for, and produced normal adults in, a susceptible lamb.