Abstract: An assembly for the attachment of an accessory to the boom of a working machine consists of a support block, quick-mounting devices attached to the block for mounting the accessory on the assembly and further on the working machine, a mounting bracket at the other end of the assembly, which mounting bracket consists of mounting devices for mounting the assembly removably on the boom or on the quick-mounting fixture attached to the boom. The support block and mounting bracket are attached to each other pivotally through spindle arms, and a power actuator is provided to swivel the support block and mounting bracket in relation to each other. To further improve the performance of the assembly, the support block consists of a rotating ring attached to the spindle arms, which rotating ring is rotatable in relation to the spindle arms, and further includes a support frame attached to the rotating ring, to which support frame the quick-mounting devices are attached.