Oswald Theodore Avery (1877-1955) was a physician, medical researcher and early molecular biologist. He was born in Halifax, Nova Scotia, but the major part of his career was spent in the United States at the Rockefeller Institute Hospital, New York City. Avery was one of the first molecular biologists and was a pioneer in immunochemistry , but he is best known for his discovery in 1944 with his co-worker Maclyn McCarty that DNA is the material of which genes and chromosomes are made. Previously, heredity information (genes) was thought to be stored in cells in protein molecules. (Before McCarty he had worked on this topic with Collin Macleod.)

This was the continuation of work by Frederick Griffith in 1928. Alfred Hershey and Martha Chase continued this work in 1952 with the Hershey-Chase experiment. These experiments paved the way for Watson and Crick's discovery of the structure of DNA, and thus the birth of modern genetics and molecular biology.

The experiment was a simple one in concept. Bacteria are able to transfer genetic material through a liquid medium to other bacteria, transforming (giving new genetic characteristics) them. Therefore if bacteria transform when an organic molecule is placed in liquid with them, then that molecule holds genetic information. For the experiment, a bacteria called a Pneumococcus was used. The R strain is harmless, while the S strain causes pneumonia in laboratory mice.

Substances were available to remove various organic compounds from bacteria, and if the remaining organic compounds were still able to cause R strain bacteria to transform then the substances removed couldn't be the carrier of genes. S strain bacteria first had the large cellular structures removed. Then they were treated with protease enzymes, which removed the proteins from the cells before the remainder was placed with R strain bacteria. The R strain bacteria transformed, meaning that proteins didn't carry the genes for causing the disease. Then the remnants of the S strain bacteria were treated with a deoxyribonuclease enzyme which removed the DNA. After this treatment, the R strain bacteria no longer transformed, as evidenced by mice not showing signs of pneumonia when injected with the bacteria. This indicated that DNA was the carrier of genes in cells.

Avery wrote in a letter to his brother, "It's lots of fun to blow bubbles but it's wiser to prick them yourself before someone else tries to."

Nobel laureate Joshua Lederberg stated that Avery and his laboratory provided "the historical platform of modern DNA research" and "betokened the molecular revolution in genetics and biomedical science generally."