Sera from seven patients from whom a C. trachomatis serovar L2 strain was isolated were tested in vitro for their ability to neutralize the infectivity of this organism. In one patient an inguinal lymph node was culture positive, whereas the remaining six patients had positive rectal biopsies. Sera from four of the patients, including the patient with the lymph node isolate, failed to neutralize serovar L2(434). In addition, the homologous strain recovered from the inguinal lymph node was available and was resistant to neutralization by the homologous sera. However, the same sera effectively neutralized a trachoma serovar, E(Bour). All four sera had inclusion immunofluorescent-antibody titers to C. trachomatis serovar L2 of 2,048 to 16,384 and microimmunofluorescent-antibody titers to the lymphogranuloma venereum biovar were equal or higher in all cases than to the 12 serovars of the trachoma biovar. The three remaining sera, while neutralizing the infectivity of the L2 strains tested, neutralized serovar E to a greater extent. These sera had the same inclusion immunofluorescent antibody titers as the sera that failed to neutralize serovar L2. To see whether this difference in the sensitivity of the biovars toward neutralization could be characterized, sera were obtained from mice immunized with different doses of both serovars L2 and E. Sera obtained from mice immunized with serovar E were able to effectively neutralize the homologous strain. In contrast, neutralization of the immunizing strain, L2(UCI-20), was not seen with sera obtained on days 7, 14, and 21 after immunization from animals receiving 8 x 105 and 8 x 104 inclusion-forming units of L2(UCI-20); however, these same sera neutralized serovar E. However, with a higher immunizing dose of L2 (107 IFUs), both E and L2 were neutralized with sera obtained 7 and 14 days after immunization. Therefore, the relative resistance to neutralization by serovar L2 compared with that of serovar E in the mouse model was inoculum dependent. All stock strains were raised in either HeLa 229 cells or McCoy cells and frozen in a sucrose-phosphate glutamic acid buffer (2-SPG) at -70°C (6). Clinical isolates were collected, placed in 2-SPG, and cultured on McCoy or HeLa 229 cells. To obtain high titers, strains were passed at least four times on HeLa 229 or McCoy cells before being frozen in 2-SPG at -70°C. Clinical isolates were serotyped by microimmunofluorescence (MIF) with monoclonal antibodies (11).Chlamydia serology. Sera obtained from patients were assayed by two different indirect immunofluorescent assays. The MIF assay used elementary bodies of the 15 serovars of C. trachomatis as the antigens as previously described (5). In the other method, which will be referred to as the inclusion indirect immunofluorescence assay (IFA), acetone-fixed McCoy cells infected with serovar L2(434) were used (5). Dilutions of sera in phosphate-buffered saline (0.01 M, pH 7.4) were placed on the antigen-containing slides and allowed to incubate at 37°C for 30 min, followed by washing...