The microcalorimetric measurement of microbial activity of biofilm samples easily allows to test the efficacy of biocides. This has been demonstrated in expts. with biofilm samples consisting of sulfate reducing (SRB) and chemoorganotrophic (COT) bacteria formed in batch culture on mild steel coupons. Addnl., biofilms were produced in continuous culture on the surface of a flow-through gold tubing in a measuring cylinder of the calorimeter. The biofilm samples were treated with the biocides formaldehyde, tetramethylammonium hydroxide, 1,8-dihydroxyanthraquinone, and a com. biocide with glutaraldehyde as one of the active compds. at varying concns. and incubation times. For evaluation of biocide efficacy microbial activity was monitored and cell counts were detd. All biocides were able to reduce microbial activity, but cell nos. did not decrease correspondingly. Formaldehyde exhibited the best effect.