Biotechnology companies have engineered plants to produce recombinant proteins for therapeutic drugs and vaccines.  Chlorogen, Inc. located in St. Louis, Missouri, inserted the protective antigen (PA) gene from Bacillus anthracis into tobacco (Nicotiana tabacum) chloroplasts to produce an anthrax vaccine.  Protective antigen is the primary immunogen of human vaccines for Anthrax disease in the United States and United Kingdom. The objective of a greenhouse study was to determine the optimum soil water content for producing PA in tobacco leaves.  Tests were conducted from December 2004 to March 2005.  Transgenic PA tobacco seedlings were transplanted to native soil (fine-loamy, mixed, thermic Typic Argiudolls) in the greenhouse floor. Treatments consisted of soil water potential irrigation thresholds (-20, - 34, -48, and -62 kPa).  Soil moisture sensors were buried 10 cm and micro-sprinklers used for irrigation. A ratoon system with two harvests was used. Results showed that irrigating pharmaceutical tobacco too frequently was as detrimental to vaccine protein production as not watering plants often enough.   The -34 kPa soil water potential treatment produced the highest leaf yield and PA concentrations.  Using ELISA quantification, the -34 kPa irrigation treatment averaged 529 µg PA kg^-1 leaf fresh weight for the two harvests.   Other treatments averaged 163 to 261 µg PA kg^-1 leaf fresh weight.