8Cl-cAMP and 8Cl-AD activity in the presence of IBMX or ADA. MM.1S (A) or MM.1R (B) cells are treated with either control medium, medium containing 3 μmol/L 8Cl-cAMP, or medium containing 3 μmol/L 8Cl-AD for 5 days and cell number is assayed and presented as described in Materials and Methods. Where indicated, IBMX is added concurrently with treatment medium at a concentration of 50 μmol/L. Where indicated, ADA is added concurrently with treatment medium at a concentration of 2 U/mL.