[Eco-toxicological study conducted with a battery of biological and phytological tests on sediments carried out on a series of 24 tributaries of the Po in 1994 and 1995]. In spring and autumn 1994 and 1995 affluent water and bed sediment were sampled from 24 tributaries of the Po river, always at the same site and at the nearest place to the confluence. In the laboratory the pore water was separated from the particle fraction of the sediment. The organic compounds bound to the latter component were extracted with solvents and brought to water solution by means of dimethyl sulfoxide. The observed animal species, along with the seeds of Lepidium, were exposed to effluent water, to pore water and to water solutions of the organic compounds extracted from bed sediment. Toxicity was evaluated on the basis of 1) direct lethality, 2) the delay of embryo development, 3) the impairment of regeneration, in the animal species, while the germination index was used for the Lepidium susceptibility. The results of these investigations demonstrate that 1) the challenged species cover a broad range of sensitivities toward environmental toxins, 2) toxicity found in river samples appears almost exclusively bound to the sediment, 3) the noxious effects found in the tributaries of the Po river increase moving downstream, and 4) likewise the sediment-bound toxicity varies among the different samplings, both as a consequence of changes in rain-dependent river flow, and because of the man-made interventions on the river sides and bed.