The productivity of American workers was somewhat lower last quarter than estimated previously, the government reported yesterday - heightening the rise in unit labor costs, which is a major factor in exacerbating inflation.

Revised figures published by the Labor Department showed output per manhour, or worker efficiency, in the private business sector up at a 4.5 per cent annual rate between July and September, rather than the 4.9 per cent pace reported earlier.

The revisions pushed the rise in unit labor cost, a major element in the wage-price spiral, to an annual rate of 3.9 per cent, up from a 3.3 per cent pace recorded earlier estimates. Real compensation per hour rose at a 3.1 per cent annual rate.

Nevertheless, the increase in unit labor costs was the smallest since the third quarter of 1975, in the wake of the deep recession. In the April-June qaurter this year, unit labor costs rose at a 7.5 per cent annual rate.

The figures came as simultaneously, the department reported that productivity in nonfinancial corporations, a manpower measure of worker efficiency, rose at a 3.8 per cent annual rate last quarter, compared to a 1.6 per cent race in the April-June period.