Mycograb (NeuTec Pharma plc) is a human genetically recombinant antibody against fungal heat shock protein 90 (HSP90). Antibody to HSP90 is closely associated with recovery in patients with invasive candidiasis who are receiving amphotericin B (AMB). Using in vitro assays developed for efficacy assessment of chemotherapeutic antifungal drugs, Mycograb showed activity against a wide range of yeast species (MICs against Candida albicans [fluconazole {FLC}-sensitive and FLC-resistant strains], Candida krusei, Candida tropicalis, Candida glabrata, and Candida parapsilosis, 128 to 256 μg/ml). Mycograb (4 or 8 μg/ml) showed synergy with AMB, the fractional inhibitory index being 0.09 to 0.31. Synergy was not evident with FLC, except for FLC-sensitive C. albicans. Murine kinetics showed that Mycograb at 2 mg/kg produced a maximum concentration of drug in serum of 4.7 μg/ml, a half-life at alpha phase of 3.75 min, a half-life at beta phase of 2.34 h, and an area under the concentration-time curve from 0 to t h of 155 μg · min/ml. Mycograb (2 mg/kg) alone produced significant improvement in murine candidiasis caused by each species: (i) a reduction (Scheffe's test, P < 0.05) in the mean organ colony count for the FLC-resistant strain of C. albicans (kidney, liver, and spleen), C. krusei (liver and spleen), C. glabrata (liver and spleen), C. tropicalis (kidney), and C. parapsilosis (kidney, liver, and spleen) and (ii) a statistically significant increase in the number of negative biopsy specimens (Fisher's exact test, P < 0.05) for C. glabrata (kidney), C. tropicalis (liver and spleen), and C. parapsilosis (liver). AMB (0.6 mg/kg) alone cleared the C. tropicalis infection but failed to clear infections caused by C. albicans, C. krusei, C. glabrata, or C. parapsilosis. Synergy with AMB, defined as an increase (Fisher's exact test, P < 0.05) in the number of negative biopsy specimens compared with those obtained using AMB alone, occurred with the FLC-resistant strain of C. albicans (kidney), C. krusei (spleen), C. glabrata (spleen), and C. parapsilosis (liver and spleen). Only by combining Mycograb with AMB was complete resolution of infection achieved for C. albicans, C. krusei, and C. glabrata.