testing one's abilities; demanding

challenging and rewarding employment

the current challenging economic environment

invite (someone) to engage in a contest

he challenged one of my men to a duel

organizations challenged the government in by-elections

dispute the truth or validity of

it is possible to challenge the report's assumptions

administer an immunogenic or infectious agent to an animal or person, in order to study the resulting immune response or measure the efficacy of a vaccine

the mice were challenged with the influenza virus