Radiation suit

Doc Brown: "Wha-what's this thing I'm wearing?"

Marty: "Well, this – this is a radiation suit."

— Doc Brown asking Marty what his older self was wearing, after seeing the tape.

A radiation suit was a yellow or white garment worn to prevent exposure to radiation. It consisted of a full body suit – with a large radioactive warning symbol printed on the back – and a helmet with a breath filter and large flaps that enveloped the wearer. Doc Brown used a white one when handling the plutonium he obtained. Any time he inserted a pellet into the fuel chamber of the DeLorean time machine, he would don the suit.

Doc had an extra yellow suit for Marty McFly which he wore during the plutonium procedure. Before Marty could take it off, the Libyans arrived and killed Doc, thus Marty kept it on all the way to 1955.

As he exited the vehicle after crashing into the Peabody family's barn, they believed him to be an extra-terrestrial and shot at him – even when he took it off – because he had "mutated into human form", according to Sherman Peabody.

After escaping, he continued to wear the suit up to where he saw the undeveloped Lyon Estates. He changed into his vest and pants and hid the car.

Later, when he needed to convince his father George to take Lorraine to the dance, he put on the suit and attached to the belt a hair dryer (as a 'ray gun') and his Walkman (as a "brain-melting" torture device) to achieve the full effect of "Darth Vader".

When George's book A Match Made in Space was published, an alien figure wearing an outfit similar to the radiation suit was on the cover.