A method for radioactive labelling of bacteria is described. [35S]-L-methionine labelled Staphylococcus aureus and Escherichia coli used as model microorganisms were adsorbed onto the surface of acrylhydrogel coated charcoal. On the average 33% of S. aureus and 10% of E. coli supplied were bound. In both cases most of the pathogens (75-90%) could be desorbed by intensive washing, but about 3% of the bacteria provided remained fixed to the surface of the charcoal. Of these remaining bacteria 50% (in the case of S. aureus) or 1% (in the case of E. coli) could be detected in subsequent cultures. These experiments demonstrate that a modified technique of haemoperfusion may be useful in the diagnosis of septicaemia. Haemoperfusion charcoal in an extracorporal circulation of a patient serves as a matrix for adsorption of microorganisms from the circulating blood. This leads to an "in vivo" enrichment of circulating pathogens which may be cultured, identified and their antibiotic sensitivity tested in the usual manner. Thus, this procedure may provide a new diagnostic tool in the diagnosis of septicaemia (9).