Wash. Rev. Code § 19.86.060
It shall be unlawful for any corporation to acquire, directly or indirectly, the whole or any part of the stock or assets of another corporation where the effect of such acquisition may be to substantially lessen competition or tend to create a monopoly in any line of commerce.
This section shall not apply to corporations purchasing such stock solely for investment and not using the same by voting or otherwise to bring about, or in attempting to bring about, the substantial lessening of competition. Nor shall anything contained in this section prevent a corporation from causing the formation of subsidiary corporations for the actual carrying on of their immediate lawful business, or the natural and legitimate branches or extensions thereof, or from owning and holding all or a part of the stock of such subsidiary corporations, when the effect of such formation is not to substantially lessen competition.
In addition to any other remedy provided by this chapter, the superior court may order any corporation to divest itself of the stock or assets held contrary to this section, in the manner and within the time fixed by said order.
RCW 19.86.060