Two agglutinating IgM mAb against mannan Ag of Candida albicans strains were investigated for their specificity. The agglutinating patterns of both mAb with a panel of stationary phase of yeast cells of standard strains did not match those of any known polyclonal antibody (PAb) factors. The reactive patterns of both mAb for a given panel of 202 isolates of seven Candida species and the mode of competitive binding between mAb and a PAb factor, as determined by a combination of direct and indirect immunofluorescence staining, demonstrated that the two mAb were a part of PAb factor 4 (a prescribed reactive pattern of adsorbed PAb) but were different from each other in their specificity. The thereby designated mAb 4b and 4c were tested under PAb factor 4-positive Candida strains and further division into two to three serotypes of each species were made. 1H-Nuclear magnetic resonance spectra (500 MHz) of purified neutral mannans from strains of each serotype showed that although the 1H-nuclear magnetic resonance spectra of mannans from two subtypes of C. albicans serotype A and from each of two serotypes of Candida guilliermondii and Candida glabrata were identical or similar, the two mAb were still able to distinguish their fine determinant structures. Our findings suggest that mAb with entirely identical specificity cannot be produced against even the same determinant groups of mannan. In addition, the fact that microheterogeneity may occur without limit in the mannans of the strains suggests that antibodies with unlimited diverse specificities are produced directed against these antigenic varieties as well.