The capacity of the beta-Proteobacterium Pseudomonas sp. strain P51, which degrades chlorinated benzenes, to metabolize 1,2,4-trichlorobenzene (TCB) under environmental conditions was tested by its release into two experimental systems. The first system consisted of laboratory scale microcosms which were operated with and without the addition of TCB and which were inoculated with sludge from a wastewater treatment plant. The second system consisted of a non sterile, water saturated soil column. We determined survival of strain P51 after its introduction and its ability to degrade TCB. The population dynamics was followed by selective plating and applying the polymerase chain reaction (PCR) to detect strain P51 and the chlorobenzene (tcb) genes on catabolic plasmid pP51. The results showed a completely different behaviour of strain P51 in the two habitats under the applied conditions. In the soil column the P51 bacteria inoculated the entire area and their population reached 2 x 10(6) cells/g soil. The population remained active since TCB was degraded to concentrations below the detection limit of 30 micrograms/l. In the sludge microcosms, the number of strain P51 cells immediately decreased from 4 x 10(7) cells/ml to 10(5) cells/ml over a period of 2 days after inoculation, and then the strain disappeared to levels below our detection limit (10(3)-10(4) cells/ml). In the reactor without TCB the population of P51 maintained a stable value of 10(5) cells/ml during 8 days but then also decreased to levels below the detection limit. In addition, no significant TCB degradation was found in the sludge reactors. The influence of presence of TCB on maintenance of strain P51 in the two habitats is discussed. This work demonstrates the possibility to successfully apply preselected strains to degrade otherwise poorly degradable substances in complex mixed microbial communities. However, survival and activity may depend strongly on the type of system into which the strain is introduced.