U Zlatého klíče, Cultural monument in Karlovy Vary, Czech Republic

U Zlatého klíče is a four-storey row house at Lázeňská 21/3 with a nine-axis facade and a five-axis roof extension. The ground floor once opened through an arcade with flat lintels on marble-clad pillars, later filled by shops. The upper floors have pilasters and cornices, and the first- and second-floor windows carry triangular and semicircular pediments on brackets.

The building was constructed between 1835 and 1836, initially functioning as a dye works before becoming a salt production facility under master craftsman Bernard Richter. This early industrial use shaped the economic history of the town.

The house was already recorded as Goldener Schlüssel in 1788, when it still served as a dye works. Spa physician Karl David Becher later kept a private museum here with a regional library, Karlsbadensia and a collection of artistic bookbindings. Spa doctor Leopold Fleckles had his practice in the house, and Karl Marx visited him during his cures.

The house stands at Lázeňská 21/3 in the spa centre of Karlovy Vary, a row building of four floors and nine window axes. The ground floor once opened onto an arcade with marble-clad pillars, later filled with shops; the entrance is therefore at street level. The five-axis roof extension is sheet-metal clad.

The Karl Marx Museum that opened here in 1960 was the first museum in the world dedicated to Marx. It occupied the house where the spa doctor Leopold Fleckles had his practice and where Marx came for treatment. The exhibition closed in 1990.