The C12 detonator, lot OM-1-23, caused malfunction of the M25A2 grenade in three widely separated instances. At Cherry Point, North Carolina, and at Twenty nine Palms, California, failures occurred with the CS1 load. At China Lake, California, failures occurred with six-month surveillance samples loaded with TIARA, formula 5. No leakage of the TIARA 5 load into the detonator cavity was detected in any surveillance samples. Pine Bluff Arsenal, Pine Bluff, Arkansas, advised that malfunction was caused by defective Mk 39 A1 primers, and that these are being replaced by the EX 2926A Western Cartridge design primers in a new procurement. Accelerated surveillance of the EX 1 Mod O Illuminating Grenade will be started on receipt of the new primer.