The exchange said in April that it was teaming with a group of high-profile broker dealers and exchange members to launch its ISE Stock Exchange unit.

The new investors join Bear Stearns, Citadel Derivatives, Deutsche Bank, Interactive Brokers, JPMorgan, Knight Capital and Sun Trading as minority owners of the new ISE Stock Exchange. New York-based ISE will retain majority ownership of unit.

The original investors contributed $32 million in capital to the project and will provide order flow and liquidity. Details of the most recents investments have not been disclosed, but according to press reports, the three firms are investing a total of $11 million.

In total, the firms are investing $43 million for a 49% stake in the stock exchange, while ISE will retain a 51% share in the venture.

David Krell, ISE's president and CEO, says the new investment "underscores our belief that the ISE Stock Exchange will provide an innovative platform for equity trading".

"The ISE Stock Exchange will provide an innovative platform for equity trading. As a result of its differentiated product offerings and unique structure, we believe the ISE Stock Exchange will not only be valuable to investors who trade equities but also will improve the dynamics of the equities market as a whole," adds Krell.

The new stock exchange will launch with its first product, called MidPoint Match (MPM), early in Q3. This is a continuous, instantaneous, fully automated and anonymous matching platform which will automatically sell equities at the midpoint price of the national best bid and offer throughout the day.

ISE says the patent-pending MPM trading platform will match and execute all round lot orders - large and small - and is especially suited for orders generated by algorithms.

MidPoint Match will be followed by an integrated displayed market offering in the fourth quarter, subject to regulatory approval.