Nov. 21 (Bloomberg) -- Turkey’s benchmark ISE National 100 Index fell 3.2 percent to 52,752.15 at the close in Istanbul, its biggest drop in a month. Of the 100 stocks on the index, 97 fell, two gained and one was unchanged.

The banking index plunged 3.9 percent, capping a six-day losing streak, as Turkey’s antitrust regulator said it was opening an investigation into 12 banks on possible collusion on setting interest rates for loans and deposits.

The following stocks were active. Stock symbols are in parentheses.

Afyon Cimento Sanayi AS (AFYON TI), a cement maker owned by Italcementi SpA’s Ciments Francais, added 2 percent to 104 liras after trading up as much as 13 percent in intraday trading. Cimsa Cimento Sanayi & Ticaret AS (CIMSA TI) said it was bidding for a 51 percent stake in the company. Cimsa shares fell 2.8 percent to 7.74 liras.

Eregli Demir & Celik Fabrikalari TAS (EREGL TI), or Erdemir, Turkey’s largest steel producer, fell 4.5 percent to 3.22 liras. Arcelor Mittal has considered selling its 26 percent stake in the Turkish steelmaker as it disposes of non-core assets, Mergermarket reported.

Yapi & Kredi Bankasi AS (YKBNK TI), the bank part-owned by Italy’s UniCredit SpA, dropped for a fifth day, losing 5.5 percent to 2.92 liras, its lowest since November 2009. The bank was cut to “hold” at Deutsche Bank AG and was among the 12 banks being probed for possible anti-competitive practices.

--Editors: Linda Shen, Alex Nicholson

To contact the reporter on this story: Benjamin Harvey in Istanbul at bharvey11@bloomberg.net

To contact the editor responsible for this story: Gavin Serkin at gserkin@bloomberg.net