The avermectin antiparasitics were discovered as a result of a collaborative agreement between Merck & Co Inc, in the US and the Kitasato Institute (now part of Kitasato University) in Japan. From a batch of 54 microbial isolates sent from the Institute to Merck in March 1974, one proved to have particularly potent anthelminthic activity – crude extracts containing unknown amounts of the unknown active principle were active when incorporated into the diet of mice at a concentration of just 3 ppm. The active components were the avermectins, while the microorganism producing them was a new species of Streptomyces which was named S. avermitilis [1]. It is extremely rare – in the following five years, Merck screened 250,000 microbial cultures for activity against either Heligmosomoides bakeri or the model roundworm C. elegans; S. avermitilis was not found in any of the samples [2].