In the largest interstate merger to date, Home Savings of America acquired six troubled savings and loan associations in Texas and Illinois. The transaction, arranged by the Federal Savings & Loan Insurance Corp., increases the assets of the nation's largest savings and loan to more than $15 billion.

Home, a subsidiary of H.F. Ahmanson & Co. of Los Angeles, now has branches stretching across the country. Just last month federal regulators approved its acquisition of thrifts in Missouri and Florida.

The Texas S&Ls merged into Home yesterday were Republic of Texas Association in Houston, Buffalo Savings and Loan of Houston, Royal Federal Savings and Loan of Dallas, El Centro Federal Savings and Loan of Dallas, and Civic Savings and Loan of Irving. The sixth institution was Hyde Park Federal Savings and Loan of Chicago.

The merger announcement said that financial assistance provided by the FSLIC--the part of the Federal Home Loan Bank Board that insures thrifts--would consist of "protection from losses related to real estate now owned by the acquired associations and future losses on any real estate acquired through foreclosure of the existing mortgage loans held by the acquired associations."

On the same day that the fifth interstate merger of savings and loans was made public, the fourth involuntary merger of savings banks in recent months was announced by the Federal Deposit Insurance Corp. Western New York Savings Bank in Buffalo was merged into Buffalo Savings Bank. The FDIC paid the acquirer $30 million in cash to compensate it for the depreciation in Western's assets.

Buffalo, with combined assets of $5.5 billion, becomes the fourth-largest savings bank in the country. Just last month, Buffalo acquired Union Dime Savings Bank in New York City in another assisted merger.

Besides effecting mergers, the bank board took other measures this week to enable more savings and loan associations to survive these difficult times. The federal agency again lowered net worth requirements, this time from 4 percent of liabilities to 3 percent.

Net worth is the surplus of assets over liabilities, the equivalent of earnings at mutual S&Ls. Last year all S&L earnings were down approximately $5 billion. By lowering net worth requirements, the regulators will subject fewer S&Ls to involuntary mergers. The last revision, from 5 percent to 4 percent, was made in November 1980.

Bank board officials estimated that between 70 and 80 S&Ls would benefit from the change because their net worth ratio is currently between 3 percent and 4 percent. It declined to reveal how many savings and loans have net worth below the statutory limit of 3 percent.

An analysis of midyear statistics issued by the bank board indicates several hundred S&Ls may be below 3 percent currently. However, creative bookkeeping cuts down the number. S&Ls are allowed to average liabilities for one or more of the past four years in determining net worth. Thus, for example, if an S&L had liabilities of $200 million in the current year, but could average with the preceding year's $150 million, its $10 million net worth would equal 5.7 percent instead of 5 percent.

At a meeting late Thursday, the regulators took several other measures to aid thrifts. Henceforth, S&Ls will be allowed to borrow without limitation in the private sector instead of being required to do half their borrowing in the Federal Home Loan Bank System. This is intended to give them greater flexibility to manage liabilities and to sell loans in the secondary market.

The board also agreed to allow S&Ls to engage in certain forms of leasing that are the functional equivalent of consumer lending.

In related news, nine depository institution trade associations met yesterday for the first time to discuss the problems of their industries. Those represented included commercial banks and bank holding companies, credit unions, savings and loan associations and mutual savings banks. At a press conference following the two-day meeting, the group said it did not try to arrive at a consensus on which legislative proposals to support.

Sen. Jake Garn (R-Utah), chairman of the Banking Committee, last month urged the financial institutions to reach agreement before hearings resume in February on comprehensive reform of the banking system. Yesterday the trade associations threw the ball back in Congress' court, saying consensus should be reached there.