Phytohaemaglutinin (PHA, a lectin derived from Phaseolus vulgaris) is routinely used as an in vitro T-cell mitogen (e.g. Avramis et al. 2001; Carloni et al. 2001). In this assay PHA is presented to cultured lymphocytes and has the rapid and easily observed effect of stimulating T-cell proliferation. Such T-cell proliferation is almost certainly an important component of the early phase of the acquired immune response in vertebrates: the assay therefore provides a clear, logical connection between the speed/magnitude of an immune response (T-cell proliferation) and host fitness in the face of pathogenic challenge. Not surprisingly this assay has consequently become popular with field ornithologists interested in assaying ‘immunocompetence’ in the context of ecotoxicology and parasite-mediated sexual selection. However, ecologists use the assay in vivo (see Goto et al. 1978) and usually perform it by injecting phosphate buffered saline (PBS) into one wing web and PHA dissolved in PBS into the other wing web. A set time after challenge the size of the swellings resulting from both injections are assessed and the measure of PHA-induced swelling is adjusted with reference to the PBS-only swelling.