The effect of 'spectrum alternation' (i.e. an alternation of strong and weak vibrational bands) in multiline pulsed first-overtone CO laser spectrum, which is determined by two sets of cascade transitions operating independently, is experimentally observed. The influence of laser mixture composition and gas density on the 'spectrum alternation' is discussed. Lasing on the highest experimentally observed vibrational transition 38 yields 36 can be obtained with a nitrogen free laser mixture or with a little nitrogen admixture only. This phenomenon can be explained by quasi- resonance collisional interaction between the 38 yields 36 vibrational transition of CO molecule and the 0 yields 1 vibrational transition of N2. A formation of first-overtone CO laser spectrum corresponding to the highest vibrational transitions is also discussed.