Chicago Mayor Jane Byrne today signed up more than 300 recruits to replace the city's striking firefighters and said she would no longer negotiate with the strikers.

"I will not talk to irresponsible men again ever," the mayor said as the strike entered its fifth day. "There is going to be no getting back together. sThere is not."

Byrne said she would fire the "hard core" leaders of the strike -- numbering between 400 and 600 of the city's force of 4,350. The recruits, who now begin a two-week, on-the-job "crash course," will replace the workers who are dismissed, the mayor said.

The unprecedented decision to hire new workers to replace strikers is a move that would have seemed inconceivable a year ago, in the city's then-stable business-labor environment.Byrne's move came as both sides in the bitter dispute seemed to harden. The strikers have repeated their determination to defy court back-to-work orders and $40,000-a-day fines.Union president Frank Muscare said he would rather go to jail than pay a fine.

Chicago's 3.5 million residents have depended on makeshift fire protection since the walkout began Thursday. Fire department officials say no more than 550 of the city's force have been on duty at any one time since then.

Bryne said today that fire protection in the city was at "dangerously low levels."

The first fire death in the city since the strike began was reported today when 37-year-old Diane Williams was found in her basement apartment after fire swept through a frame bungalow on the South Side.

But fire department officials said their response to the fire call had been normal. "I can't blame the death on the strike," a department spokesman said.

Bitterness was vented at the scene of another blaze overnight. A group of men fighting a fire at a North Side mattress factory was taunted and cursed by strikers who stood by, looking on.

"We won't forget this, you scab," one striker yelled. "Don't be chicken," others screamed. Another group laughed as a volunteer battling the blaze fell on the ice.

As more than 300 men crowded a fire department gymnasium this morning to sign forms giving them "emergency appointments" to the department, they too were taunted as "scabs" and "strike breakers.'

Byrne and her personnel director Charles Pounian said they would try to give permanent appointments to any man who completed the two-week training course. The usual training course takes eight weeks.

The recruits' names were gleaned from department eligibility lists. Most aspire to become career firefighters.

"A fireman said he was going to bust my jaw if I crossed the picket line," Howard Zelenka, 23, told United Press International. "But they aren't paying me, are they?"

Some potential recruits, however, declined the appointment while the strike is on.

"You're a marked man for the rest of your life," one told UPI. "If a floor falls out under me while I'm fighting a fire, they're [union members] going to have second thoughts about coming after me."

The union has been trying to negotiate its first-ever contract with the city, but has been rebuffed by Byrne because of disagreements with the bargaining unit and no-strike clauses that would govern future agreements.

Even though talks broke down Sunday. Circuit Court Judge John Hechinger, who levied the contempt penalties said the dispute was almost settled "three or four times during the day."

a written contract for the firemen was a major campaign promise of Bryne a year ago when she ran for mayor against the party organization.

At the time Muscare's union was the only one in the city to endorse her candidacy.

During a December transit strike, Byrne similarly threatened to fire union members if they did not return to work. They later heeded a court injunction and returned to work.

In addition to the South Side and North Side fires, several others struck the city over the weekend. One left 20 people homeless, another forced 80 persons to flee and a third critically injured one man.

Some 2,500 union members, some with wives and children, showed up at a noisy rally Sunday night.

There, Muscare accused the mayor of "trying to break the union" by manipulating the courts. But he promised to "give the lady peace" once a contract is signed.