HAMMOND, Ind. (AP) _ Two environmentalists scaled a chemical plant's 300-foot smokestack Wednesday to protest plans to burn hazardous waste at the facility, and a fellow activist said the climbers would remain atop the structure through the night.

The two climbed the Stauffer Chemical Co. smokestack around 5:30 a.m. and unfurled a banner reading ''Poison,'' Diane Hebert, toxics coordinator for Greenpeace's Great Lake office, said from Chicago.

Police secured an area around the plant and were waiting for the men to climb down, Sgt. Paul Savickis said Wednesday nigth.

''They're supposed to come down tomorrow at 1 o'clock,'' Savickis said. ''What happens between now and then, I can't tell you.''

Greenpeace spokeswoman Kate Karam said Stauffer has applied for a permit from the Environmental Protection Agency to burn over 300 types of hazardous wastes as a fuel source.

Tom Gruendel, Stauffer's personnel and safety manager, would not comment on the incineration plans, but read a company statement condemning the protest as ''totally irresponsible.''

The plant manufactures sulfuric acid from sulfur, regenerates used sulfuric acid and produces other sulfur-based products.

Mrs. Hebert identified the climbers as Ken Hollis, 25, of Ontario, Canada, and Steve Loper, 31, of Chicago.