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