We have tested the hypothesis that dense cell formation in sickle cell disease is associated with increased binding of calpromotin to the membrane, an event that occurs during the activation of calcium-dependent potassium transport. By SDS polyacrylamide gel electrophoresis, we found that sickle cell membranes contained more calpromotin than did normal membranes when stained with Coomassie brilliant blue or when transferred to nitrocellulose paper and immunostained with horseradish peroxidase. Also, the membranes from dense sickle cells contained significantly (P = 0.00055) higher levels of calpromotin, 2.62 Β± 1.59 Β΅g/mg membrane protein, compared to light sickle cells, 1.40 Β± 0.70 Β΅g/mg membrane protein, when measured by an enzyme-linked immunosorbent assay. The ratio of calpromotin associated with dense cell membranes to light cell membranes was significantly greater than 1.0 (P < 0.00005). Transmission electron micrographs of immunogold-labelled membranes supported the increase in calpromotin binding in dense sickle cell membranes. In addition, the immunogold probe demonstrated clustering, which was not observed in light sickle cell membranes nor in normal membranes. Finally, we incubated HbSS cells in vitro using a repetitive deoxygenation/ reoxygenation procedure to produce dense cells and then measured the levels of calpromotin associated with their membranes. As expected, the levels of calpromotin bound to the membrane doubled during the procedure relative to the basal levels at the beginning of the incubation. The correlation coefficient, calculated between the increase in dense cell formation and the increase in calpromotin associated with the membrane, was statistically significant (P = 0.038). The results demonstrate that an increase in calpromotin binding to the membrane is associated with dense cell formation presumably through the activation of the calcium-dependent potassium channel. Am.