NEW YORK (CNNMoney.com) -- Manufacturing activity grew for the eighth straight month in March, at the fastest rates since July 2004, according to a report Thursday.
The Tempe, Ariz.-based Institute for Supply Management's (ISM) manufacturing index rose to 59.6 in March from the February reading of 56.5. Economists surveyed by Briefing.com were expecting a reading of 57.
"The report is very positive on all points," said Dan Meckstroth, a chief economist with the Manufacturers Alliance. "The index itself is at its best we've seen so far in this recovery, and it's up substantially since February."
The strong number was driven by significant growth in new orders and production, as well as larger inventories, which grew for the first time in 46 months.
The breakeven point for the index is 50, with a reading above that indicating growth in the sector. March's higher score means the manufacturing sector grew at a faster pace than the month before.
The monthly report surveys purchasing managers, and because it's a survey, some analysts say the index is highly subjective and overrated. But others view the report as a valuable indicator of broad trends and the overall health of the economy.
Overall, 17 of the 18 manufacturing industries surveyed showed growth. Plastics and rubber products is the only industry that reported contraction.
The new orders component rose to 61.5 from 59.5 the month before, while the production component increased to 61.1 from 58.4.
Inventories inched higher to 55.3 -- the first time that component has posted above the 50 tipping point in 46 months.
While the report's employment component slipped 1 point to 55.1 in March, that was still above the level that indicates growth.
Strong productivity gains usually come at the early stages of recovery, Meckstroth said, as new orders bounce back, manufacturers start revving up their previously underutilized machinery, and factories that may have been overstaffed start adding work hours. Job growth follows suit a bit later in the recovery, he said.
"As production picks up there's more pressure on companies to add people," Meckstroth said. "The fact that we're getting any growth in employment is very positive. It's another positive sign that the industry's recovering and will continue to recovery."