BACKGROUND: The manufacture of high-yield pulps is attractive due to its efficient utilization of lignocellulosic material. However, their rapid brightness reversion when exposed to heat and light restricts their more widespread use in high-quality papers. This study investigated the potential of bio-treatment with more than 30 species of fungi and yeasts to inhibit heat-induced yellowing of high-yield pulps. RESULTS: Treatment with Penicillium sp. ZCF34, Saccharomyces cerevisiae INVS2 and 724 reduced the brightness reversion and increased the brightness of wheat straw semi-chemical pulp after 4 days incubation (postcolor number decreased by 33.96%, 66.42% and 57.09%, and brightness increased by 0.84%ISO, 0.48%ISO and 0.89%ISO, respectively). Phonerochaete chrysosporium W14 treatment was best for depressing yellowing of bleached aspen CTMP (chemi-thermo-mechanical pulp)and Saccharomyces cerevisiae AH22 treatment gave the highest brightness and a lower postcolor number. For unbleached aspen CTMP, treatment with Fusarium concolor X4 and Soccharomyces cerevisiae H158 was effective in improving brightness and brightness stability after 2 and 4 days incubation, respectively. CONCLUSION: Treatment with some microorganisms increased pulp brightness and strongly suppressed heat-induced yellowing. It offers an option for improving the brightness stability of high yield pulps and some valuable information for bio-pulping and bio-bleaching with microbial treatment. (C) 2009 Society of Chemical Industry