Abstract We have investigated the chemical structure of the antigenic determinant of Candida krusei cell-wall mannan. Acetolysis of the mannan, obtained by extraction with alkali and purified as copper complex, gave five oligosaccharides (from mono- to pentasaccharide) and a minor amount of an octasaccharide. Partial acetolysis of the mannan gave a large amount of mannooctaose. We have examined the inhibition by these oligosaccharides of the precipitin reaction between anti- Candida krusei serum and homologous mannan, and found that the mannooctaose was the most effective inhibitor. Results obtained by proton magnetic resonance spectroscopy, methylation analysis, and other structural studies of Candida krusei mannan, suggest that the octasaccharide possesses six α(1–2) linkages and one α(1–6) linkage located in the middle of the chain, and that this mannooctaose may be responsible for the specificity of Candida krusei cell-wall mannan.