Leucaena (Leucaena spp., especially L. leucocephala) is an arboreal, tropical legume that ranges into the cool subtropics and equatorial elevations up to 1,000 m. One of its uses includes forage for livestock, but introduction of leucaena outside its indigenous range often has led to acute and chronic toxicosis. The major toxic constituents of leucaena are the nonprotein free amino acid mimosine and its ruminal degradation product, 3-hydroxy-4(1H)-pyridone (3,4-dihydroxypyridine; 3,4-DHP). Leucaena also contains appreciable quantities of condensed tannins. In ruminants, mimosine is a depilatory agent and 3,4-DHP is a potent goitrogen. In the 1980s, Australian workers demonstrated that the geographical limits of leucaena toxicosis were due to the absence of ruminal bacteria capable of degrading 3,4-DHP, and successfully introduced 3,4-DHP degrading ruminal bacteria from a Hawaiian goat into goats and cattle in Australia. Simple in vitro screening methods have been developed for detection of 3,4-DHP degraders in ruminal samples and feces. Also, several strains of 3,4-DHP degrading ruminal bacteria have been characterized and have been given the genus and species designation, Synergistes jonesii. Ruminal inoculation with ruminal contents from adapted animals, enriched cultures of 3,4-DHP-degrading ruminal bacteria, and pure cultures of S. jonesii have all been used successfully to establish ruminal populations that are capable of degrading 3,4-DHP and preventing leucaena toxicosis.(ABSTRACT TRUNCATED AT 250 WORDS)