The cholera toxin-colloidal gold-labeled IgG-F(ab')2 anticholera toxin ultrastructural immunocytochemical procedure was used for the localization of GM1 monosialoganglioside on the surface of human blood cells. The number of gold particles per micron of cell surface was counted and the data subjected to statistical analysis. Cholera toxin (CT) binding characteristics assessed in several subjects showed consistent labeling patterns for the various hemic cells, although some quantitative differences were noted in surface labeling densities between subjects. Neutrophils were invariably the most heavily labeled of the hemic cells, while lymphocytes, erythrocytes, and platelets exhibited only limited CT labeling. Exposure of hemic cells to neuraminidase induced a major increase in surface CT labeling that proved to be directly related to cell type and differed in many respects with the CT labeling pattern noted in nonenzyme treated cells. Newly exposed CT binding sites attributed to "masked" GM1 and/or to neuraminidase-transformed GD1a or GT1 gangliosides, showed that the number of new binding sites were nearly twice as abundant on platelet and monocyte sufaces as on the surfaces of neutrophil, lymphocyte, and erythrocyte populations. However, ratios of new CT binding sites to those normally available for CT binding were approximately 10:1 for erythrocytes, approximately 3--7:1 for lymphocytes, monocytes, and platelets, and approximately 1:1 for the neutrophil group. Exogenous GM1 was incorporated into the cell surface of the hemic cells in a differential manner. Platelets showed a dramatic increase in surface CT labeling, viz. approximately 12- to 20-fold, compared to that of other hemic cells; however, neutrophil and erythrocyte GM1 uptake was limited. Our studies have demonstrated that distinct differences exist in the extent of surface CT labeling of the various types of blood cells. They further indicated that the ability of the cell surface to incorporate exogenous GM1 may represent a differential expression of the physiochemical properties of the surface of the individual cell types.