At an altitude of 2550 m a search has been carried out for the 550-mass particle which, at the Moscow International Conference in May 1956, was reported to exist with an abundance of 0.5% relative to muons of the same range. Preliminary evidence against this datum is now reinforced by the results of a new series of measurements in which particles heavier than pions but lighter than protons are selected by means of two Cerenkov counters. The mass of each selected particle is derived from measurements of energy loss in a proportional scintillator, and range in a large multiplate cloud chamber. The latter alone yields substantlal informatlon on the nature of the stopping particles. No example of a mass-550 particle has been found out of the 25 expected on the basis of the reported abundance. (auth)