Two blue cone monochromats and four rod monochromats have been studied by increment threshold measurements applying the Stiles' principle. Some rudimentary colour discrimination was reported by the blue cone monochromats. One patient showed good discrimination between short- and middle-wavelength lights in matching experiments using the Nagel II apparatus. His neutral band in the spectrum was at lambda = 485--495 nm. Dichromatic vision could not be proved in the other patient. The blue cone monochromats also had good responding blue mechanism in the periphery. Indication of cone activity other than blue cones is found in both kinds of monochromats; these cones being probably of the rhodopsin-cone type (pi0 cones). The conclusions are drawn that the inhibitory effect of the pi0 cones upon the rod mechanism may account for the differences shown by our two blue cone monochromats as to visual acuity, nystagmus and photophobia. Likewise, their differences regarding dichromatic vision may be explained by an unequal number of pi0 cones in their retinas rather than by differences in their blue mechanisms.