In the course of studies on species‐specific cell aggregation in the cellular slime molds, the specificity of heat‐stable surface antigens was investigated. With phenol–water, antigens could be extracted from freeze‐dried amoebae of Dictyostelium discoideum, the serological reactivity of which is insensitive to ribonuclease, pronase and α‐amylase; however, it is sensitive to periodate. Serological reactivity of phenol–water extracts was traced to a fraction which contains N ‐acetyl‐glucosamine, fucose, mannose and polypeptide, and behenic acid as the principal fatty acid. The polysaccharide moieties of these compounds are considered to carry the serological specificities. In comparing the two species, Dictyostelium discoideum and Polysphondylium pallidum, it was found that the non‐dialyzable fraction of the phenol–water‐extracts contain different proportions of the three sugars (mannose–fucose–N ‐acetyl‐glucosamine in D. discoideum 1:4.5:12; in P. pallidum 1:1:2). Species‐specificity of the phenol–water extracted antigens of the two species was demonstrated by precipitation with the respective antisera. With the fluorescent antibody technique and by agglutination, species‐specific antigens which are periodate sensitive could be detected on the surface of heat‐killed (2 h; 100°) cells. As revealed by agglutination inhibition, these heat‐stable cell surface antigens are identical, at least in part, with the phenol–water extracted antigens. Copyright © 1969, Wiley Blackwell. All rights reserved
