A group of thirty-five mothers and their babies at parturition were examined by the in vitro lymphocyte transformation test to determine sensitization by oral bacterial antigens, B-cell mitogens and dental plaque. Lymphocytes from babies of sensitized mothers with gingival or periodontal disease gave the highest frequency (70 and 63%) and magnitude (mean stimulation index of 3.4 and 3.3) of response in cultures stimulated by Actinomyces viscosus and Veillonella alcalescens. However, IgM antibodies to V. alcalescens antigen were absent from cord sera. With one exception, stimulation of lymphocytes from babies of unsensitized mothers with clinically healthy gingiva was not found with these antigens. The response of cord lymphocytes from mothers with gingival or periodontal disease to antigens from oral bacteria, as compared with the response of cord lymphocytes from mothers with clinically healthy gingiva, seemed specific, since a corresponding difference in response to unrelated antigen PPD was not found. The response of cord and maternal lymphocytes to B-cell mitogens was also determined. Maternal lymphocytes responded in the following decreasing order of effectiveness: dextran sulphate, levan, lipopolysaccharide and dextran B1355; whereas cord lymphocytes were stimulated in the reverse order of effectiveness.