A method which allows the investigation, in an atomic beam, of noble-gas lines having a resonant lower level is discussed : a population inversion is produced by means of a first laser between an excited level and the resonant level of interest; a second tunable laser induces transitions between those two levels. The experiment has been done on a Kr beam; the expected amplitude and width of the resonance have been calculated and are compared with the experimental data. The method has been applied to isotope-shift measurements and gives : ν(86Kr) - ν(84Kr) = (118 ± 6) MHz and ν(84Kr) - ν( 82Kr) = (108 ± 6) MHz in the line λ = 5 87 nm (1s4-2p2) of KrI