The impact of four PCB congeners: 3,3 ' ,4,4 ' -tetrachlorobiphenyl (lUPAC congener #77), 3,3 ' ,4,4 ' ,5-pentachlorobiphenyl (IUPAC #126), 2,2 ' ,4,4 ' ,5,5 ' -hexachlorobiphenyl (lUPAC #153) and 3,3 ' ,4,4 ' ,5,5 ' -hexachlorobiphenyl (IUPAC #169) was investigated on the reactive oxygen species (ROS) production by coelomocytes of the echinoid Para-centrotits lividus, an important species in marine benthic, ecosystems. PCBs were found to increase ROS production and to delay the time of peak production. These effects were stronger on bacteria-stimulated cells and were congener-specitic: coplanar congeners (#77, 126 and 169) had more effect than the non-coplanar PCB #153. Among coplanar congeners, PCB #169 showed dose-dependent effects, whereas PCB #77 and 126 were more toxic at high and low doses, respectively. The relative immunotoxicity of the different PCB congeners is discussed in the light of their structural properties and biological affinities. (C) 2001, Elsevier Science Ltd. All rights reserved.