Of all his research and experimentation, Chang's work in "in vitro" fertilisation was arguably his greatest achievement. In 1935, Gregory Pincus had claimed to have achieved successful mammal birth from the result of "in vitro" fertilisation of rabbit eggs. As nobody, including Chang, could repeat this feat at the time, doubts were cast over the authenticity of the claim. Then finally, in 1959, Chang "in vitro" fertilised a black rabbit's eggs with a black rabbit's sperm, transferred them to a white rabbit, and was able to produce a litter of young black rabbits. This was the sort of evidence attesting to the feasibility of "in vitro" fertilisation for which many scientists had been searching. In the years that followed, Chang and his associates conducted further research to determine specific conditions of successful "in vitro" fertilisation as well as to perform the technique on other mammals such as hamsters, mice, and rats. It was on the basis of Chang's findings that the first "in vitro" fertilisation of human eggs was performed, leading to the birth of the world's first "test tube baby" in 1978.