Abstract
An electron microscope cytochemical reaction, based on the specificity of concanavalin A for certain sugar residues, was used to study the surface coat of cells in cultures of an Adenovirus 12 hamster tumour cell line (TAd~12~ XIV), of cell strains from other hamster tumours induced by simian Adenovirus 7 (SA~7~) and of secondary cultures of normal hamster embryo cells. When the reaction was performed on living cells, in situ in the bottles in which they had been grown, a uniform surface coat of positive material was found on the normal embryo cells. The tumour cells were also positive, but the reaction product had a tendency to be more patchily distributed over the surface. This discontinuity of the labelling was particularly pronounced in the strain TAd~12~ XIV, but was less visible in the highly malignant strains SA~7~. When the reaction was performed on cultures previously fixed with formaldehyde, no difference was found between the three types of cell cultures. Simple stereological analysis confirmed these findings. It was concluded that loss of material occurred differentially between the different types of culture during the stages of the reaction before fixation. The possibilities of this loss being due to physico‐chemical differences in the cell surface material or to underlying metabolic changes are discussed.