Prime rate slips to 17 1/2% at two more big US banks

Two more major banks lowered their prime lending rate by half a point, to 17 1/2 percent, amid indications that interest rates would continue to decline. The Banks, Chase Manhattan, the country's third-largest, and First National Bank of Chicago, the ninth- largest, matched the 17 1/2 percent rate set Friday by Chemical Bank.

Other large banks are also expected to lower their prime rate, the interest banks charge on loans to their best corporate customers. Prime lending rates are expected to fall even further as inflationary pre ssures continue to ease, industry analysts said.