Isolates of ADV replicate to rather high quantities in lungs from neonatally infected mink kits. The virus was analysed for polypeptide composition, and for the first time high molecular weight polypeptides have been observed in in vivo produced ADVs. These polypeptides are analogous to those of in vitro produced ADVs. The molecular weights of the structural polypeptides of the low virulence Pullman ADV and the highly virulent DK and Utah I isolates of ADV were found to be 88kD and 78kD and in vivo produced ADV-G polypeptides were found to be 85kD and 75kD, the same molecular weights as those described for in vitro produced ADV-G. Presence of the ADV coded, non-structural polypeptide with the molecular weight of 71kD (p71) was also demonstrated in the lung tissue from mink kits.