4-Chlorobiphenyl biodegradation was monitored using several methods including molecular detection, microbiological and ecotoxicological techniques and analytical tools. Experiments were carried out in microcosms with artificial soil and an uncultivated sandy loam soil, contaminated at a rate of 1 g kg(-1) of 4-chlorobiphenyl. Two strains were used for bioremediation: a 4-chlorobiphenyl degrader, the bacterial strain Pseudomonas sp, strain B4 and the bacterial strain Pseudomonas sp, strain CBS3 which is able to mineralize 4-chlorobenzoate. Molecular tools were constructed for the detection and the quantification of dihydroxybiphenyl dioxygenase gene (bphC) from Pseudomonas sp. strain B4 and for the detection of the 4-chlorobenzoate coenzyme A dehalogenase gene from Pseudomonas sp. strain CBS3. Evidence was found for 4-chlorobiphenyl toxicity to barley and earthworms using standardized ecotoxicological tests. When Pseudomonas sp. strain B4 was introduced alone into microcosms, no decrease in toxicity occurred although 4-chlorobiphenyl was entirely degraded. The introduction of Pseudomonas sp. strain CBS3 into inoculated microcosms resulted in the disappearance of the 4-chlorobenzoate accumulated during the degradation of 4-chlorobiphenyl by Pseudomonas sp. strain B4, and in the disappearance of any toxic effect indicating that remediation was complete. Similar results were obtained independent of the nature of the soil used in microcosms. These results underline the need for taking biological effects into account in order to assess remediation efficiency.