Snake handling is an extremely rare activity practiced by a very small number of Pentecostal churches - most in the southern, rural United States - on the basis on Mark 16:17-18: "And these signs will accompany those who believe: in my name they will cast out demons; they will speak in new tongues; they will pick up serpents with their hands; and if they drink any deadly poison, it will not hurt them; they will lay their hands on the sick, and they will recover" (ESV).
It is important to note that the originality of the last 12 verses in the book of Mark are highly disputed, even by many Christians.
Yet, even those who advocate for the passage's inclusion into the Gospel, don't also argue that snake-handling is present-day application of the text for Christians, Pentecostal or not.
The practice of snake-handling
Practitioners believe snake handling is a demonstration of their faith in God. While advocates of snake handling come out of Pentecostal churches, no major Christian denomination, Pentecostal or otherwise, supports the practice.
One of the original leaders of snake handling in the United States, George Hensley, broke away from the Church of God (Cleveland, Tennessee), because of his allegiance to the doctrine.
Most snake handlers are found in Alabama, Georgia, and Kentucky, Tennessee, and West Virginia. Some states such as Alabama, Kentucky, and Tennessee, have made the activity illegal, punishable by fine, but it still occurs because practicing churches are mostly in rural settings.
Congregations where snake handling occurs tend to be very small. Snake handlers have reportedly been bitten on numerous occasions. Some have even died. Notably, in 1998, evangelist John Wayne Brown, died after being bitten while handling a snake in an Alabama church. His wife also died from a snake bite.