Nitrogen metabolism in the rumen affects both the efficiency of ruminant production and the environmental impact of excreta from ruminant livestock production. Inefficient N retention by rumen microorganisms is compensated in production terms by feeding excessive amounts of dietary protein to the animal to meet required output levels. This leads directly to the excretion of N-rich wastes. Microbial protein synthesis in the rumen is the major source of amino acids entering the small intestine and available for absorption in ruminants [1, 4]. However, microbial protein turnover in the rumen may result in the net microbial protein outflow being less than half the total protein synthesised [9]. In vitro studies suggest that engulfment and digestion of bacteria by protozoa is by far the most important cause of microbial protein turnover in the rumen, with autolysis, other lytic factors and endogenous proteolysis being of minor importance [11]. Thus, it is apparent that removing ciliate protozoa from the rumen (defaunating) should avoid the recycling of nitrogen between bacteria and protozoa and thereby increase the efficiency of nitrogen metabolism in the rumen and stimulate the flow of microbial protein from the rumen [12]. A variety of techniques to remove protozoa from the rumen has been tested experimentally, but none is used routinely, because of toxicity problems, either to the rest of the rumen microbial population or to the host animal [12]. Recently, there has been an increased interest in plant secondary metabolites for use as possible defaunating agents. Here we describe three methods for screening plant material for antiprotozoal activity.