121 (West Riding) Medium Regiment RA(TA) was formed by the conversion of 121 Field Regiment in January 1944. It went into suspended animation in April 1946 and was re-constituted as 321 (West Riding) Heavy Anti-Aircraft Regiment RA(TA) in January 1947.
| Jan 44 | Formed from 121 Field Regiment | Felixstowe, Suffolk, UK |
| Jan 44 | 8 Army | Italy |
| Feb 44 | Home Forces | UK |
| Apr 44 | 2 Army | UK |
| Jun 44 | 5 Army Group RA | NW Europe |
Batteries
| Jan 44 | 275, 276 |