نبذة مختصرة : When flagellated organisms are exposed to homologous anti-flagellar serum, they are im-mobilized. This is readily demonstrated macro-scopically by application of the Gard technique (Gard, 1937; Gard and Erickson, 1939). During an antigenic study with certain of the gram negative, non-fermenting bacteria (Menolasino and Goldin, 1954), the Gard technique for flagel-lar immobilization was employed. While a marked increase in the percentage of non-motile colonies in the presence of specific anti-flagellar serum was observed, it occurred to us that it would be of interest to determine, by flagellar staining, what the actual immobilization effect was due to. Are the flagella merely immobilized without any apparent morphological change, or are they al-tered morphologically in some way so that motility is impaired? To our knowledge, no direct, visual investigation of the possibility that immobilization is accompanied by concur-rent alteration of flagellar morphology has been done. MATERIALS AND METHODS One strain of gram negative rods for each of the major types of flagellation described by Houwink and Van Iterson (1950) and Leifson (1951) was selected. All cultures were checked by the usual bacteriological procedures to confirm their identity and purity. These types are described in table 1. Organisms used for the preparation of antigens for immunization were plated on 0.3 per cent semi-solid agar. The colonies showing the highest degree of motility were picked into broth and the motility and flagellation were ascertained by hanging drop and flagellar staining. With those strains that were satisfactory, immunizing anti-gens were prepared in the routine manner. The resulting antisera were tested for H and 0 ag-
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