The chemical and serological properties of the full, naturally occurring empty and artificially produced empty particles of foot-and-mouth disease virus, serotype A(subtype 10, strain 16) have been studies. The full 146S particles comprised the virus RNA, three polypeptides (VP1 to VP3) mol. wt. about 30 X 10-3, one polypeptide (VP4) mol. wt. about 13-5 X 10-3, and a small amount of a polypeptide (VPo) mol. wt. about 43 X 10-3. The naturally occurring 75S empty particles contained no RNA and much less VP1 and VP4 than were found in the fall particles. However they contained a much greater proportion of VPo than the full particles. Dialysis of purified full particles against tris-EDTA, pH 7-6, produced artificial 75S empty particles which contained only a small amount of RNA and no VP4; otherwise the polypeptide composition was similar to that of the full particles. Immunological and serological tests showed that the full particles were antigenically similar to the naturally occurring empty particles but distinct from the artificial empty particles. The latter particles, however, had serological properties similar to those of the 12S protein subunit of the virus. Both the full and naturally occurring empty particles attached efficiently to susceptible cells, whereas the artificial empty particles attached only to a limited extent. The results are related to the function of the individual polypeptides of the virus particle and compared with published work on other picornaviruses.