Serratia marcescens O antisera prepared with boiled bacterial suspensions gave relatively low titers when tested with boiled broth cultures in tube or tray agglutination tests, but extremely high titers when tested in the haemagglutination test with conserved sheep erythrocytes coated with alkali-treated heat extract. However, an antiserum prepared with boiled O group 6 bacteria, did not react at all. A proper O6 antiserum could only be prepared if the O6 cells were not heated at 100 degrees C. The antigenic relationships between standard strains of S. marcescens were investigated by means of haemagglutination tests. The antigenic relationships already described in the literature were confirmed. Additional reciprocal relationships were observed, some of which could be explained by 8 antigenic factors which are designated I-VIII. O antisera could be made monospecific by absorption. It was confirmed that standard strain O6H3 has two antigenic determinants which should be written as O6ab, that standard strain O6H8 has O6bc and that O14 has O6b. Standard strain O7 was found to possess O6a in addition to the specific determinant O7, whereas the antigen O6c turned out to be identical with O8. The standard strain O14 has, in addition to O6b only the factor hitherto described as Co12, 13, 14 and no specific O14 antigen. We propose to redesignate Co12, 13, 14 as O14 and to write the formula of O12 and O13 strains possessing this antigen as O12, 14 and O13, 14 respectively. O antigen O23 should then be written as O14, 23. For similar reasons we propose to redesignate Co2, 3 as O25. The O antigen O20 was found to be related to the antigens O16ab, O16ac and O16acd in such a way that it should be written O16ae. For similar reasons O22 should be written as O10ac. When serotyping Dutch isolates of Serratia marcescens, hitherto undescribed antigenic combinations such as O2, 16ab; 03, 6a; O3, 21 and O4, 6b were encountered. For H antigen typing we found the slide agglutination with cultures grown on semisolid medium the most appropriate.