Schistosoma mansoni cercariae mechanically transformed into schistosomula were injected, either dead or alive, into the tail vein (2.000 larvae/0,15 ml) of Balb/c mice which were either previously infected with S. mansoni (10 weeks/50 cercariae) or non-infected. Histological examination of the lungs 24, 48, 72 and 96 hours after injection revealed that inflammatory reaction around schistosomula occurred only in the groups injected with dead schistosomula (killed by freezing and thawing). In non-infected animals the reaction was predominantly macrophagic while in those infected many eosinophils appeared around the dead larvae. These results are at variance with those obtained in vitro and suggest that in vivo the participation of eosinophils in the schistosomulum induced reaction in sensitized animals is secondary to the death of the larvae