Various techniques were examined to determine optimum conditions for exsheathing infective larvae of 3 important ruminant parasites (Haemonchus contortus, Ostertagia ostertagi, and Trichostrongylus colubriformis). In repeated experiments, aliquots of 10(5)-10(6) infective larvae, 1-2 mo old, of each parasite were incubated in each of 4 exsheathing media (distilled water, Earle's balanced salt solution + carbon dioxide, nematode washing buffer + carbon dioxide, or sodium hypochlorite) for 1 or 18 hr. In each case, the percentage of larvae exsheathed and infectivity for jirds was determined. Results of these studies indicate that no single exsheathing technique of those studied is optimum for every parasite. In addition, caution must be used in drawing conclusions from in vitro studies using exsheathed larvae because techniques that routinely provide high percentages of exsheathment also appear to reduce viability.