We have analyzed the interaction of adamantyl Gb 3 (adaGb 3 ), a semi-synthetic soluble analog of Gb 3 , with HIV-1 surface envelope glycoprotein gp120. In this analog, which was orginally designed to inhibit verotoxin binding to its glycolipid receptor, Gb 3 , the fatty acid chain is replaced with a rigid globular hydrocarbon frame (adamantane). Despite its solubility, adaGb 3 forms monolayers at an air-water interface. Compression isotherms of such monolayers demonstrated that the adamantane substitution resulted in a larger minimum molecular area and a more rigid, less compressible film than Gb 3 . Insertion of gp120 into adaGb 3 monolayers was exponential whereas the gp120/Gb 3 interaction curve was sigmoidal with a lag phase of 40 min. Adding cholesterol into authentic Gb 3 monolayers abrogated the lag phase and increased the initial rate of interaction with gp120. This effect of cholesterol was not observed with phosphatidylcholine or sphingomyelin. In addition, verotoxin-bound adaGb 3 or Gb 3 plus cholesterol was recovered in fractions of comparable low density after ultracentrifugation through sucrose-density gradients in the presence of Triton X-100. The unique biological and physico-chemical properties of adaGb 3 suggest that this analog may be a potent soluble mimic of Gb 3 , providing a novel concept for developing GSL-derived viral fusion inhibitors. -Mahfoud, R., M. Mylvaganam, C.A. Lingwood, and J. Fantini. A novel soluble analog of the HIV-1 fusion cofactor, globotriaosylceramide (Gb 3 ), eliminates the cholesterol requirement for high affinity gp120/Gb 3 interaction.