Upon establishment of optimum analytical conditions for the forward mutation assay, various types of soy sauces manufactured in China, Korea and Japan (37 samples) and Japanese natural extracts (9 samples) were measured for their mutagenicity with the following results : (i) accurate determination on histidine-rich samples could not be carried out by the standard Ames's preincubation method; (ii) most of the mutagenic activities in soy sauces were eliminated by treatments for removal of free histidine using Blue rayon or Extrelut column; (iii) no interference with histidine on mutagenicity determination of histidine-rich samples was observed when measured by the forward mutation assay proposed originally by Skopek et al (Proc Nat Acad Sci U S A 75 (1978) 410-414) using Salmonella typhimurium TM677, when correction was employed for the loss of colonies by high salinity samples; (iv) mutagenicity was found in all soy sauces (+S9, 26-72 (average 46) mutants mg-'; -S9, 51-111 (average 81) mutants mg-') and half of the natural extracts tested (+S9, 1350-1450 (average 1400) mutants mg-', -S9, 1150-2650 (average 1693) mutants mg-') by the forward mutation assay. The structures, origins and variability of mutagenic compounds during the manufacturing process would require further study.