Humoral responseswere examined in idiopathic juvenile periodontitis (IJP) including antibody titers to representative strains of the five groups of periodontosis‐associated bacteria. Titers as determined by indirect immunofluorescence were compared in IJP, non‐IJP family members, periodontitis and periodontally healthy subjects. Serum concentrations of IgG, IgA, IgM and IgE were also assayed. Antibody titers to the periodontosis‐associated bacteria were generally low. Some disease specificity was observed in that titers to Groups II and III bacteria were highest in IJP. These titers in IJP were significantly higher (P<0.01) than in the other subjects. Titers to Group I bacteria were similar in all periodontal groups except the periodontally healthy subjects where titers were the lowest. Titers to Group I bacteria were significantly higher (P<0.01) than in the periodontally healthy subjects. Titers to Groups IV and V were uniformly low in all periodontal groups.Immunoglobulin concentrations of IgA, IgM and IGE were similar in IJP and non‐IJP. The IgG concentrations in IJP, while within the normal range were significantly higher in IJP compared to non‐IJP family members.