We investigated antigenic relationships between human calicivirus (HCV) strains and Norwalk virus by using immune electron microscopy (IEM) and radioimmunoassay (RIA). Three serologically distinct HCV strains, UK1, UK2, and Japan, were demonstrated by IEM, as was evidence for two additional strains, UK3 and UK4. Although HCV strains and Norwalk virus were distinct by IEM, 12 of 20 patients with gastroenteritis due to HCV UK4 and two of eight with gastroenteritis due to UK2 showed seroconversions to Norwalk virus by RIA. These naturally occurring antibody responses in humans, as detected by RIA, support the concept that Norwalk virus belongs to the family Caliciviridae. An RIA for HCV Japan antigen also detected HCV UK1, UK2, and UK4 and thus appears to identify a group-specific antigen for these viruses. An RIA for antibody to HCV Japan failed to identify seroconversions in 45 of 47 patients with gastroenteritis due to HCV UK. These results may reflect different reactivities of various immunologic tests for the identification of infections due to small gastroenteritis viruses.