Periodically appearing amorphous aggregates, 'marine snow', are formed in the sea and if settled as mats on the sea bottom cause death of benthic metazoans. Especially those animals are killed which are sessile filter feeders, e.g. sponges, mussels, or Anthozoa. The etiology of the toxic principle(s) is not yet well understood. Gel-like marine snow aggregates occurred in the Northern Adriatic during summer 1997. Samples of these aggregates were collected during the period July to September and the outer as well as the inner zones were analyzed for (i) cell toxicity, and (ii) chemosensitizing activity of the multixenobiotic resistance (MXR) mechanism. Organic extracts were prepared and cell toxicity was determined using mouse lymphoma cells. The experiments revealed that the major activity is seen in the center of the mats of the gel-like aggregates; a growth inhibitory activity of up to 54% (correlated to 5 ml of snow sample) was determined. The same extracts were used to determine the inhibition of the P-glycoprotein (Pgp) extrusion pump which confers the multixenobiotic resistance. The analyses were performed with cells from the sponge Suberites domuncula and with gills from the clam Corbicula fluminea in situ. Both systems have been shown to express the Pgp extrusion pump. The data show that extracts from the outer zone of the gel-like aggregate samples display pronounced inhibitory activity on the MXR extrusion pump and hence act as chemosensitizers by reversing the MXP property. These findings indicate that gel-like aggregates contain compounds in the outer zone, chemosensitizer of the Pgp extrusion pump, which lower the level of protection of metazoan animals towards dissolved compounds in their surrounding milieu, and in the center toxic compounds which are--very likely--even in the absence of chemosensitizers hazardous for the invertebrates.