The spectral sensitivities of the colour vision mechanisms, as measured by the increment threshold technique, were examined for groups of ten normals, ten deuteranomals and five deuteranopes. Only in a few cases did the abnormal curves lie outside the normal range, but statistical analysis allowed a high level of significance to be placed on differences between the mean values for each group. The deuteranomals' "green" π4 mechanism appeared to be shifted about 10 nm towards the red end of the spectrum, while the deuteranopes' π4 was normal in the blue region but had the spectral sensitivity of the "red" π5 in the long wavelengths. The "high intensity state" π4′ was severely depressed in the deuteranomals and by an amount independent of the change in π4, while π4′ in the deuteranopes suffered a localised loss of sensitivity in the middle of the spectrum. For this reason it is proposed that π4′ and π5 be considered mechanisms rather than high intensity states. The deuteranopes also showed an exaggeration of the long wavelength π1 inflexion, and a loss of the secondary maximum of π2, suggesting that these characteristics are in some way dependent on π4.