Milbemycin A4 oxime is a derivative of milbemycin A4 and a component of milbemycin oxime , compounds that both have insecticidal and nematocidal activity. Milbemycin A4 oxime (0.05 mg/kg) reduces the number of microfilariae of the heartworm D. immitis in naturally infested dogs. It inhibits the growth of clinical isolates of C. glabrata with MIC80 values ranging from 16 to greater than 32 μg/ml. Milbemycin A4 oxime (2.5 μg/ml) blocks efflux of fluconazole from a clinical isolate of C. glabrata, but not from a strain lacking the efflux pumps CgCDR1 and PDH1, and reduces the MICs of fluconazole and 4-nitroquinoline 1-oxide in wild-type C. glabrata. It enhances adriamycin-induced inhibition of cell growth, as well as increases the intracellular accumulation of adriamycin and the P-glycoprotein substrate rhodamine 123 , in adriamycin-resistant, but not -sensitive, MCF-7 breast cancer cells in a concentration-dependent manner.