'Calcium' was a British commando intelligence mission to the Czechoslovak resistance movement (3/4 April 1944).

Undertaken at the same time as 'Barium', the 'Calcium' mission involved the parachute delivery of Jaroslav Odstrčil, Josef Gemrot, Frantisek Siroký and Karel Niemczyk. The party was flown from Brindisi in southern Italy and successfully parachuted into the area of Nasavrky, though one of the party’s three equipment containers was lost. The party travelled to Jihlava, but were denied safe refuge at several addresses and then divided, Siroký and Niemczyk making for Novému Městu, where they established contact with the local element of the Moravian resistance, on 21 May established radio contact with London, and then moved to Proseč.

Odstrčil and Gemrot became involved in a firefight with the Gestapo in Netín on 23 June, Odstrčil being killed but Gemrot managing to escape despite being wounded. Recovering from his wounds, Gemrot continued to collect and radio information to London.

Late in 1944 and early in 1945, the 'Calcium' mission was instrumental in the reception of the 'Tungsten', 'Platinum-Pewter' and 'Bauxite' missions, and May 1945 the group was involved in negotiations with the German garrison in Nasavrky. The members of the 'Calcium' party then moved to the Moravian highlands to bolster the 'Tungsten' party, and its members were later seized by the advancing Soviet forces but subsequently released.