Lloyd H. Conover, a chemist whose breakthrough invention of one of the most effective and widely prescribed antibiotics, tetracycline, led to a whole new approach to developing such drugs, died on Saturday in St. Petersburg, Fla. He was 93.

His death was confirmed by his son Craig.

Dr. Conover started his research at Pfizer in Brooklyn in 1950, when pharmaceutical companies, spurred by the success of penicillin against battlefield infections during World War II, were racing to find new antibiotics. Most early antibiotics were naturally occurring chemicals produced by microorganisms that lived in soil or on decaying fruit; the strain of penicillin used to protect wounded soldiers came from a moldy melon.

Few scientists at the time thought it was possible to improve on antibiotics, which they viewed as a result of an evolutionary process that had equipped microorganisms with the ultimate chemical weapons to survive.

Dr. Conover, however, became intrigued with two naturally occurring antibiotics that, except for two atoms, were chemically identical. To Dr. Conover, the atoms seemed out of place. Each antibiotic had a chlorine or oxygen atom where he expected to find a hydrogen atom. Would swapping in hydrogen improve the potency of the drugs?