The necessity to combat oil slicks makes it necessary to assess the relative effectiveness and toxicity of dispersal agents used. This is usually done by using individual or groups of indicator organisms. The theory behind the various approaches is outlined, and some considerations are listed for the choice of technique. 2 organisms were chosen - the Portuguese oyster (Crassostrea), and the phytoplankton alga Phaeodactylum tricornutum Bohlin, on which if feeds - and the oyster closing reaction, and level of inhibition of growth under the influence of irritant products was measured. Methods are described, and the results shown for emulsifying, agglomerating and precipitating products. The results are discussed, and the mode of interference of the products suggested. Products were allotted a 'coefficient of effectiveness', and one emulsifier, 5 agglomerants and 4 precipitants are concluded to be of use. It is noted that they may have different effects on other species.