High numbers of advanced stages of Oesophagostomum radiatum developed from exsheathed, infective larvae in complex, cell-free Medium API-1, with or without the addition of glutathione (reduced) or a bovine heme in the form of "unbound" hemin or the bound hemin component of Fildes' reagent (a peptic digest of defibrinated bovine blood). Based on culture periods of 28 to 52 days, O. radiatum larvae developed to fourth molt in Medium API-1, with and without a bovine heme compound. However, in medium containing bovine heme, development continued to young adult males and females within 25 to 35 days. The most advanced stages attained were young adult males and females in which mature reproductive systems showed the beginning of spermatogenesis and oogenesis. The optimal culture system for development to young adults required Medium API-1 supplemented with 20 mM glutathione (reduced) for the first 7 days, followed by nonsupplemented medium for 7 days, and thereafter medium supplemented with Fildes' reagent which contained hemin in a concentration of 48 micrograms/ml of medium. In this system, O. radiatum developed to early, young adults in 25 days and to "late," young adults by 37 to 52 days. The largest young adult male and female were 8.3 and 9.0 mm long, respectively. This system produced maximal yields of up to 25% young adults in populations of about 80,000 O. radiatum by days 28 to 42.