The growth of Frankia spp. strain ORS 020607 in BAP medium was studied by using two methods simultaneously: determination of Bradford protein content and INT (2-(p-iodophenyl-3-(p-nitrophenyl)-5-phenyl tetrazoliumchloride) reduction activity (IRA). With the latter test, red formazan crystals formed intracellularly were extracted with methanol. Colouration intensity was estimated by absorbance spectrophotometry at 490 nm. The protein content and IRA of the culture were monitored for 96 days. IRA appeared to reflect the 'metabolically active' biomass of Frankia more accurately than the Bradford protein estimations.