Endotoxin of Gram-negative bacteria has been regarded as one of the important pathogen to produce periodontal inflammation. As conventional rats (CV) are naturally stimulated by bacteria of their microflora and germfree rats (GF) are not, existence of difference in antibody production can be expected between them. The purpose of this study is to assess the extent of antibody production against endotoxin (E. coil 0111: B4) in CV rats and in GF rats. Endotoxin (E. coli 0111: B4, 0.1mg/0.2ml) was injected intraperitoneally (ip) into each rat and serum was obtained from the jugular vein. Antibody titration was measured by the passive hemagglutination test on microplate using endotoxin coated sheep blood cells (SRBC).<br>The following findings were obtained:<br>1. When rats were injected with endotoxin every week, body weight of CV rats decreased from the day of the first injection to 2 weeks and then slowly recovered to the initial value. On the other hand, in the case of GF rats, endotoxin injected rats increased body weight after the first injection and then suddenly stopped increasing it on the 2nd measurement (i.e. after the 3rd injection).<br>2. Titer of anti-endotoxin serum reached to the maximum (2⁸) after 5th to 7th injection of E. coli 0111: B4 in CV rats. Titer of 2⁴ were obtained in serum of GF rats that had 5th to 7th injection.<br>3. Two series of serum of 4 CV rats showed titer of 2¹ and 2² against E. coli 0111: B4 coated SRBC, but any significant titer against endotoxin was not obtained in GF rats.<br>4. When specificity of anti-E. coli 0111: B4 sera was examined by estimating adsorption with E. coli 0111: B4 and with other endotoxins, any antibody titer against E. coli 0111: B4 was not detected in both CV and GF rats after the sera had the adsorption process with E. coli 0111: B4. Only one exception in each group existed.<br>Decrease of titer in CV rats and no significant change in GF rats was obtained when the sera were pretreated by other endotoxins. Although any titer could not be detected in GF rats, significant antibody titer against 0127, 0128 and 055 endotoxin was recognized in anti-E. coli 0111 sera derived from CV rats.<br>Results indicated that anti-E. coli 0111: B4 sera of CV rats contain heterologous antibody produced by injection of E. coli 0111: B4. Anti-sera of GF rats showed specificity against E. coli 0111: B4. GF rats were useful for estimating immunological mechanisms of the periodontal disease.