The processing of the high-mannose asparagine-linked oligosaccharides synthesized by first-trimester human placenta has been investigated. Tissue was pulsed for 1 h with [2-3H]mannose and chased for zero, 45, 90, and 180 min in media containing unlabeled mannose. Glycopeptides, prepared by Pronase digestion of the delipidated membrane pellets at each time point, were treated with endo-beta-N-acetylglucosaminidase-H to release the high-mannose asparagine-linked oligosaccharides. The largest major processing intermediate isolated was Glc1Man9GlcNAc, which was converted into Man9GlcNAc, and then into Man8GlcNAc, Man7GlcNAc, Man6GlcNAc, and Man5GlcNAc. There was also a minor pathway in which mannosyl residues were removed prior to the glucose. By carrying out the detailed structural characterization of the individual processing intermediates, it was possible to demonstrate that processing of the Man9GlcNAc to Man5GlcNAc proceeded by the nonrandom removal of the alpha 1,2-linked mannosyl residues. Specifically, of 12 possible sequences of removal of the four alpha 1,2-linked mannosyl residues present in Man9GlcNAc, first-trimester human placenta utilized only two of these in the processing of asparagine-linked oligosaccharides. It is suggested that the limited number of processing pathways reflects a high degree of specificity of these reactions in human placenta.