The effect of increasing the length of the 3'-untranslated region (UTR) on expression of luciferase mRNA was examined in transiently transfected carrot protoplasts. The effect of the 3'-UTR on both poly(A)- and poly(A)+ mRNA was examined. A nested set of constructs in which the 3'-UTR increased from 4 to 104 bases was generated by the introduction and reiteration of a 20 base sequence downstream of the luc stop codon. For poly(A)- mRNA, there was a consistent increase in expression when the length of the 3'-UTR was increased from 4 to 104 bases. For poly(A)+ mRNA, expression increased 18-fold when the length of the 3'-UTR was increased from 7 to 27 bases. Further increases in the length of the 3'-UTR did not affect expression. The increase in expression was largely due to an increase in translational efficiency. These data suggest that the length of the 3'-UTR plays an important role in determining the extent to which a poly(A) tail can stimulate the translation of an mRNA.