The invention provides competitive immunoassay techniques for high sensitivity detection of delta-9-tetrahydrocannabinol (cannabis; THC) employing a carrier conjugate of an intermediate in the biosynthesis of cannabis, more particularly 5-pentylresorcinol conjugated to a macromolecular carrier via its hydroxyl groups. By employing such a conjugate with anti-THC antibody in a lateral flow immunochromtography test device convenient on-site testing for low levels of cannabis in liquid samples may be achieved. Such testing is particularly favoured for roadside testing for cannabis in oral fluid samples.