Sodium butyrate induced adhesion of cultured mastocytoma p-815 cells to the surface of a standard tissue culture grade petri dish. The ratio of the number of adherent cells to that of total cells (adherent plus floating cells) was dependent on the serum concentration and on the dose of sodium butyrate. During approximately the first 6 hr after the addition of sodium butyrate, no cells adhered. The optimum conditions for adhesion were provided by 2 mM sodium butyrate and 15% fetal calf serum, 44 hr after addition of this compound. Morphologically, adherent cells consisted of spindle-shaped and round cells: the latter clustered to the former. Low concentrations of actinomycin D (0.005 microgram/mL) and of cycloheximide (0.5 microgram/mL) inhibited cell adhesion. Adherent cells were easily detached by 0.25% trypsin-0.02% EDTA but not by EDTA alone. Adherent mastocytoma cells which were cultured in the presence of 2 mM sodium butyrate, re-adhered to the surface of the dish. The ratio of adhesion in the second dish, however, was very low (35% after 2 hr incubation). Radioactive iodinated surface proteins of butyrate-treated adherent cells showed two new bands (70,000 and 92,000 D) which were not detected in control cells, but there was no difference in the extent of labeling of high molecular weight protein (250,000 D) between butyrate-treated and control cells.