The microcalorimetric measurement of microbial activity of biofilm samples allows easy testing of 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. Only 3% of the original microbial activity remained, and a redn. in cell nos. of five orders of magnitude in the case of SRB was measured. In contrast, tetramethylammonium hydroxide had only a slight effect. Microbial activity was reduced only to 20%, and the cell nos. did not decrease at all. The other biocides exhibited intermediate effects. In general, cell nos. of chemoorganotrophic bacteria in these biofilm samples decreased more than did the nos. of SRB. If the biocide contg. medium was substituted by a biocide-free one, regrowth and reactivation of biofilm cells resulted. However, the activity did not reach initial values within the exptl. time.