281 double crossover mutants were examined by fermentation experiments and hplc analysis for production of polyketides . besides avermectin b1a and b2a , a new compound ( about l ~ 4ug / ml ) which showed a retention time and fragmentation patterns identical to those of the authentic sample of 22 , 23 - dihydroavermectin bla by hplc and lc / ms analysis were detected in culture extracts of 27 isolates 在篩選的281個雙交換突變株中,有27株除了產生阿維菌素b1a和b2a ,還產生一個新組分,經hplc和lc ms分析,證實該組分為c - 22 , 23雙氫阿維菌素b1a (即伊維菌素b1a ) 。