A genetic variant of the H5N1 virus, termed subclade 2.3.2.1, was first identified in Bulgaria in 2010 and has subsequently been found in Vietnam and Laos. Several cases of human infections with this virus have been identified. Thus, it is important to understand the antigenic properties and transmissibility of this variant. Our results showed that although it is phylogenetically closely related to other previously characterized clade 2.3 viruses, this new 2.3.2.1 variant exhibits distinct antigenic properties and shows little cross-reactivity to sera raised against other H5N1 viruses. Like other H5N1 viruses, this variant preferentially binds to avian-type receptors, but it contains substitutions at positions 190 and 158 of HA that have been postulated to facilitate HA binding to human-type receptors and to enhance viral transmissibility among mammals, respectively. However, this virus has not acquired the capacity for airborne transmission between ferrets. These findings highlight the challenges in selecting vaccine candidates for H5N1 influenza because these viruses continue to evolve rapidly in the field. It is important to note that some variants have obtained mutations that may gain transmissibility between model animals, and the close surveillance of H5N1 viruses in poultry is warranted.