A method for treating periodontal disease by first disinfecting the affected part with a bactericidal disinfectant containing ferric ions (Fe.sup.+3) and L-ascorbic acid as the principal components and one or more of sorbic acid, benzoic acid and para-hydroxybenzoic acid esters, and then injecting a preparation containing a Lactobacillus casei into the affected part. Antibiotics are contained in the preparation, for enhancing the therapeutic effects.