The cycling of organic carbon in the deep sea was inferred from measurements of sediment trap and box core samples taken on the Biscay and Demerara abyssal plains of the North Atlantic. Of the input of organic carbon to the bottom, less than 10% was buried, i.e., not consumed biologically. Based on laboratory measurements of bacterial activity in the sediment samples, incubated under in situ temperature and pressure, it was possible to attribute at least 13 to 30% of the total inferred biological consumption of organic carbon to microbial utilization. The complementarity of results from these biochemical and microbiological measurements implies that the decompression of cold abyssal samples during retrieval efforts does not prevent meaningful experiments on the microbial inhabitants, once returned to in situ pressure.