Newly-merged US firm Piper Marbury Rudnick & Wolfe is planning to be in London within a year and may ally with a UK firm.
Baltimore's Piper & Marbury and Chicago's Rudnick & Wolfe merged on 30 October to form a 750-lawyer practice - the 11th largest in the US in terms of numbers.
Jeffrey Liss, chief operating officer of the new firm, says: "London is a particular area of interest because of our corporate and M&A practice as well as our franchising and distribution practice. The real estate practice is an increasingly worldwide market, and to some extent so is the regulatory practice.
"Our lawyers are over there all the time on those issues already, and the question is, now that we have the size and risk capital to do this, do we open up shop or look at some kind of alliance?"
Liss says international expansion is a "front burner rather than back burner" issue.
"Given the significant slice of our practice that now has an international component, it is essential that we think every day about how to maximise that," Liss says.
He does not rule out a merger with a UK firm. "I think that is all part of formulating our strategy," he says.
The firm also plans to expand into Brussels, the Far East and California. It currently has offices in Chicago, Baltimore, Washington DC, New York, Tampa, Philadelphia, Dallas and Reston, Virginia.
US firm Holme Roberts & Owen's London office is becoming a multinational partnership (MNP) to recruit English solicitors.
The nine-strong office will add an English corporate finance partner and two assistants before the end of the year.
Corporate finance partner Marty Collins Rolle says: "That is just the immediate plan to meet our immediate growth needs."
Rolle says the firm needs to practise English law to serve US clients setting up European operations. MNP status will also allow its dual-qualified lawyers to offer advice in Europe.
The Denver-based firm's London office has moved to new premises in Savile Row.