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 在筛选的281个双交换突变株中,有27株除了产生阿维菌素b1a和b2a ,还产生一个新组分,经hplc和lc ms分析,证实该组分为c - 22 , 23双氢阿维菌素b1a (即伊维菌素b1a ) 。