The construction of piles and diaphragm walls often requires the use of drilling fluids to support the excavation in the temporary case. These fluids can affect the bond of the reinforcement to the concrete and whilst this was recognised in the 1960s it is often ignored in current design. This paper discusses the design bond stresses for reinforcement (deformed bars only) cast into concrete that has been placed under drilling fluids. Two drilling fluids have been considered in the study, bentonite and a synthetic polymer slurry. The earlier concerns over the performance of deformed bars in concrete cast under bentonite are discussed and results from three sets of tests, two laboratory based and one in the field, are reviewed. The results are compared with the values given in current UK structural codes of practice. Advice for engineers designing reinforcement for structures cast under drilling fluids is given as well as recommendations for further work into the performance of these reinforcing bars. A. E. K. Jones, BEng, PhD, CEng, MICE Arup Research and Development Eur Ing D. A. Holt, BEng, MSc, CEng, MICE Arup Geotechnics