Evidence is presented for the production in dengue type 2 virus (DEN-2)-infected BHK cells of large virus-specific proteins with molecular weights up to 250 000. These proteins were most prominent in lysates of cells which had been labelled with [35S]methionine for 7 to 15 min. During pulse-chase experiments, these high mol. wt. proteins appeared to be converted into smaller, more stable, proteins with mol. wt. between 10 000 and 98 000. Finally, inhibition of proteolysis prevented the chasing of label from the high mol. wt. proteins to the smaller viral proteins which normally accumulate in DEN-2-infected cells. These findings are consistent with the idea that processing of large polyprotein precursors plays an important role in the production of flavivirus proteins.