O positive and O and A negative blood is in very short supply and donors are needed urgently, the Blood Transfusion Centre said on Tuesday.
It said in a statement that unless urgent donations were made, a number of operations will have to be cancelled.
It is the second time in three days that the centre has had to make a public request for blood donors.
Blood can be donated at the Blood Transfusion Centre in Guardamangia, which is open until 6pm on Tuesday. In Gozo, donations can be made at the Xewkija Clinic between 1 and 5pm.
Donors have to be in possession of their identity card.
The centre said people who had the common cold should leave 15 days from their last symptoms before donating blood. People who have been very sick and took antibiotics should leave four weeks before donating.
Further information is available here or on tel: 79307307, freephone: 80074313.