while a lesser concentration was detected in 13 day PI adults. However, when total RIA positive material was considered (conjugated and nonconjugated), 13 day PI adults and 6 wk old L3 had similar amounts of ecdysteroid-like substance(s). However, correlation of the concentration of these ecdysteroid-like substances with known physiological and/or biochemical events as shown for the molt and metamorphosis of insects (Shaaya and Sekeris, 1965, Gen. Endocrin. 5: 35-39) is premature. Our results indicate that investigation of the involvement of ecdysteroids in the control of growth and development of nematodes is worthwhile. The high concentration of ecdysteroid-like substance(s) in the L3 reared under axenic conditions is particularly significant. The culture medium contained no such detectable material and thus could not have served as an exogenous source. Thus, it is probable that the ecdysteroid-like substance(s) detected was synthesized by the larvae from cholesterol in the medium. Another possibility is the endogenous presence of ecdysteroids in the eggs, but this seems unlikely due to the high concentrations detected. The presence of ecdysteroidlike substances in the egg should be investigated since cholesterol was reported in the eggs of this species (Mauro and Weinstein, 1979, Int. J. Parasit. 9: 421-427). We recognize that the results obtained with TABLE I. Amounts of ecdysteroid equivalents detected in extracts* of Nippostrongylus brasiliensis and axenic culture medium by radioimmunoassay.