43 was a decision of the Supreme Court of Canada on the scope of the federal Parliament's criminal law power under section 91(27) of the Constitution Act, 1867.

Lenore Westendorp and a friend approached an undercover police officer on a street in Calgary and solicited him for sex.

Laskin C.J., writing for a unanimous Court, held that the law was ultra vires the province.

Laskin found that the law was "colourable", as its true purpose was not to keep the streets safe but to control or punish prostitution.

The court decided that what was occurring was that a municipality disapproved of prostitution, and was attempting to enact a criminal law in order to discourage it.