The solid-state fermentation (SSF) method described enabled accurate determination of variables related to biological activity. Growth, respiratory activity and production of carboxymethyl-cellulose-hydrolysing enzyme (CMC-ase) activity by Trichoderma reesei QM9414 on wheat bran was used as a model SSF. The standard deviation (s) of measured substrate weight loss and glucosamine content, and the overall standard deviation of oxygen consumption rate (OCR) and carbon-dioxide production rate (CPR) after 72 h of fermentation were less than 7% of the mean measured values. A statistical method to estimate the standard deviation of time-dependent data is presented. A closed carbon balance could be set up with the results of the measurements of CPR, elemental composition and chemical composition, indicating the reliability of the methods used. Measurements of CMC-ase activity were less accurate due to interaction between substrate and product. The measurement of the ATP level was also less accurate.