Inflation slowed last month to an annual rate of 7.5 percent, the lowest monthly increase since last July, the Labor Department said yesterday.

Slower energy price increases were largely responsible for the improvement over February, when prices went up at an annual rate of 12.1 percent, the department said. Energy costs were pushed up sharply in February as a result of the immediate decontrol of oil prices announced by President Reagan in January.

The March figure for consumer prices means that some 35 million Social Security recipients will get an 11.2 percent cost-of-living in their checks as of July. This is because the CPI rose by 11.2 percent between the first three months of 1980 and the same period this year, the base for the cost-of-living calculation.

Prices in the Washington area rose by 2 percent in the two-month period ending in March, largely because of higher housing costs, the Labor Department also reported. This was above the 1.8 percent rate for the nation as a whole in February and March.

Administration officials were cautious yesterday about the inflation numbers, which showed that prices were 10.5 percent higher in March than a year earlier. Presidential economic adviser Murray Weidenbaum called the Consumer Price Index report "good news," but added, "we can't be satisfied with an inflation rate of 10.5 percent over the past year."

In a push for the president's economic program, he said "the basic conditions which have led to the high underlying rate of inflation . . . remain with us, and there is little evidence that they can be corrected by merely continuing to follow the policies of the past."

Noted Wall street economist Henry Kaufman said Wednesday that the large across-the-board tax cut proposed by Reagan was "inappropriate" because of the danger of continued high inflation. But budget director David Stockman said yesterday that Kaufman was wrong about the inflationary effects of the president's program, and called for its enactment by Congress.

In March the unadjusted consumer price index stood at 265.1, compared to a base of 100 in 1967. This means that goods and services costing $100 in 1967 cost $265.1 in March this year.

Hourly earnings rose by 9.1 percent in the year to March, according to other figures released by the Labor Department yesterday. The purchasing power of an average hour's work, after adjusting for inflation, was down by 1 percent over the year, although it rose by 0.2 percent between February and March, the department said.

Transportation and housing costs rose by 0.9 and 0.5 percent respectively last month, the Labor Department said yesterday. This was the lowest rise in seven months in each category.

Gasoline prices, which leaped 8.4 cents a gallon in February, went up 3.5 cents last month, bringing the average price of all types of gasoline to $1.39 a gallon in March. This month there have been some declines in gasoline prices which should help moderate the overall price rise.

Other components in the March price index were:

a 0.3 percent rise in food and drink prices, following a 0.4 percent February increase.

a drop of 0.4 percent in new car prices, largely because of rebates.

a full 1 percent rise in clothing prices, with women's and girls' clothes going up 1.9 percent.

a 0.9 percent increase in medical care costs.

The rise in Social Security will give the average retired worker living alone $374 a month, up from $337. A typical couple's benefits will improve from $576 to $640 a month, and the maximum check will go up from $677 to $752.90 a month.

The minimum Social Security payment, which Reagan wants to abolish, will rise from $153.10 to $170.30 a month.

Meanwhile the Office of Management and Budget yesterday released a study which it said refuted charges that the Reagan economic program will be harsher for the Northeast and Midwest than for the South and West. According to the OMB calculations, the per capita impact of the entire program is roughly equal in all parts of the country.

However, the study lumped together both the Reagan spending cuts -- which admittedly do cut social welfare programs substantially more deeply in the Northeast and Midwest -- and the per capita benefits of the tax cuts backed by the administration. These tend to be higher in higher-wage areas -- the Northeast and Midwest.