Biofilm forming organisms are incubated to form a biofilm on projections by providing a flow of liquid growth medium across projections, the direction of the flow of liquid being repeatedly changed, and an assay made of the resulting biofilm. Biofilm forming organisms are incubated to form a biofilm on projections arranged in rows, with several projections in each row, while providing a flow of liquid growth medium across each row of projections, and an assay made of the resulting biofilm. Sensitivity of the biofilm to antimicrobial reagent may be determined by treating the projections with antimicrobial reagent before carrying out the assay, by treating each row of projections with a different antimicrobial reagent, and each of the projections in a row with a different concentration of antimicrobial reagent.