An assay for the measurement of the total biomass in an unfiltered fer
mentation sample has been developed using a simple sequential injectio
n system, with either total transmitted light or light scattered at 90
degrees as the analytical signal. The biomass is determined without p
re-treatment or dilution of the original sample. In the past, the use
of unfiltered bioprocess samples in flow injection has been avoided du
e to problems associated with a complex sample matrix, e.g., cell aggr
egation, clogging of the flow system, cells sticking to tubing walls a
nd cross contamination. The sample range for the assay is 0.2 to at le
ast 80 g l(-1) biomass with relative standard deviations of <3%. The r
esults from a single assay are available 3 min after assay initiation.