In the soil bacterium Bacillus subtilis Spx is a key regulator that controls expression, positively or negatively, of several genes in response to certain oxidative stresses that lead to the formation of unwanted disulphide bonds. Here we characterized the yjbH gene and show that it encodes a novel effector of Spx. The yjbH gene is part of the yjbIH operon that encodes a truncated haemoglobin (YjbI) and a predicted 34 kDa cytosolic protein of unknown function (YjbH). Deletion of yjbIH or yjbH has pleiotropic effects and affects growth, sporulation and competence development. Cells lacking yjbIH display a reduced sensitivity to the thiol oxidant diamide and show an apparent down- or upregulation of several transcripts that belong to the Spx regulon. Twenty-two suppressor mutations that bypass the defects conferred by yjbH were isolated. These mutations were identified as six deletions, three nonsense and 11 missense substitutions in the spx gene. Real-time quantitative reverse transcription polymerase chain reaction (qRT-PCR) analysis showed that mutations in yjbIH or yjbH do not affect the level of spx transcription. The combined data from the present work show that strains lacking yjbIH or yjbH overproduce Spx under unperturbed growth. The elevated Spx concentration cannot be attributed to an increased spx expression but is likely to result from control at the post-transcriptional level. YjbH is proposed to affect the cellular concentration of Spx by modulating proteolysis via the ClpXP protease.