The direct agglutination of erythrocytes by Neisseria meningitidis was studied as a marker for adherence. Hemagglutination (HA) was studied by slide test (5-min incubation) and by dilutions in microtitre plates (20-h incubation). Meningococci that were freshly isolated from subjects agglutinated only human cells by slide test but human, dog, rabbit, guinea pig, and rat cells were agglutinated in the microtitre system. Newly isolated strains were piliated and HA positive but pili were lost after 10 passages on agar, and bacteria became HA negative. HA could be maintained by "affinity culturing," which selected markedly adhesive bacteria: erythrocytes with adherent meningococci were isolated and cultured on agar. This procedure was repeated daily. HA titres were unaffected by mannose but were reduced by sonic disruption, trypsinization, ultraviolet irradiation, heating (65 °C), and formaldehyde. Encapsulated (serogroupable) bacteria had low HA titres compared with nongroupable strains, and purified capsular polysaccharides A and C inhibited HA. Meningococcal HA is probably mediated by pili and modified by other factors such as encapsulation. Colonial variation was not a reliable indicator of piliation, and HA is best used for this purpose.