Due to his political convictions as a member of the Bulgarian Social Democratic Workers Party (Broad Socialists), the father, Tsocho Bratanov, had difficulties securing a teaching position.

After completing 6th grade in his hometown in 1928, Kiril Bratanov moved to the capital, Sofia, where he continued his studies at the 2nd Secondary School for Men (Втора софийска мъжка гимназия), graduating in 1930.

Participants from Austria, Australia, Belgium, Bulgaria, Czech Republic, Denmark, Germany, France, UK, Hungary, Italy, Norway, Netherlands, Romania, Sweden, USA, USSR, and Yugoslavia attended the symposium.

At the 3rd ISIR Congress in 1986 in Toronto, Professor Rupert Billingham, an immunologist and one of the pioneers in organ transplantation, said: "Let us never forget that, was it not for Bulgaria, was it not for our dear friend Kiril, we would have never gathered here as a research community to deal with such an important problem for humanity, the immunology of reproduction..." In early October 1986, Bratanov participated in an international symposium in New Delhi, India.

A week after his return, while preparing his opening address to the forthcoming Bulgarian-Egyptian Symposium on Biotechnologies in Reproduction, to be held in Varna, he suffered a heart attack and passed away on 16 October at the age of 75.