Several formulas have been proposed to express the relation between the minimal phototoxic dose (MPD) and the 8-MOP plasma concentration [1, 2] . Goldstein et al [3] recently proposed another formula, viz. constant K = log. MPD x log. (8-MOP),Insnn• However, this formula is not valid in all cases and K is dependent on the units used. Indeed, K = 0 whenever (8-MOP),Iusnn• or MPD equals 1, and it becomes negative when either is less than 1. It can only be by chance, and in a limited range, that log. MPD X log. (8-MOP),I,smn is a constant.