Approximately 14 years elapsed between Sir Alexander Fleming's discovery of penicillin (in 1928) and its full-scale production for therapeutic use (in 1942) in World War II. The following factors were responsible for the delay: a scientific explanation of Fleming's "phenomenon," classification of the fungus secreting the active substance, source of the mold, initial difficulty of other bacteriologists in reproducing Fleming's discovery, identifying the chemical makeup of penicillin, search for other penicillin-producing organisms to enhance production of penicillin, purification and crystallization of penicillin, experiments on animals (chiefly mice) to determine toxicity, hesitancy to administer the drug to humans, standardization of an effective dosage for humans, and search for equipment and financial resources to enhance full-scale production. The adjunctive role of serendipity (chance, happenstance, improbability, and luck) in overcoming these obstacles and in contributing to the successful, scientific conclusion of the penicillin project is an unusual story.