One promising strategy for reducing the cost of cellulolytic enzymes is to produce them in transgenic crop plants. We have shown that transplastomic tobacco expressing an endoglucanase (Acidothermus cellulolyticus E1 catalytic domain) can yield in excess of 10% of soluble leaf protein as the desired recombinant product. Furthermore, the recombinant enzyme is readily recovered from dried plant material (60-90% recovery) in an active form. We have applied the same strategy to the expression of 3 cellobiohydrolase genes: CBH I of Trichoderma reesei, Cel6B (formerly E3) of Thermobifida fusca, and Cel7D of Phanerochaete chrysosporium. In each case, the coding sequence was modified at its 5' end by the addition of 30 bp encoding the N-terminal 10 amino acids of the highly expressed psbA gene product encoded by the plastid genome. The stability of the resulting fusion proteins was assessed in an E. coli expression system, with significant differences observed between the three cellobiohydrolases. In addition, significant growth rate inhibition was observed with some constructs when incorporated into the tobacco plastid genome. A comparison of mRNA and recombinant protein levels between transplastomic tobacco lines and E. coli expression strains will help us to gain insight into the factors affecting recombinant enzyme production in transplastomic tobacco.