Association of a 100 kD protein with CD9 on stromal cells is increased by CD9 ligation. MS-5 cells were preincubated with KMC8.8 or control antibodies for 15 minutes, biotin labeled, and then lysed before immunoprecipitation with KMC8.8. Densitometry showed that the quantity of 100 kD protein in the pellets increased an average of 2.7-fold in three experiments by ligation of cells with CD9.