The advance of pachytene chromosome mapping as a practical method in human cytogenetic analysis has been hindered by preparations of insufficient quality. Slides suitable for chromomere mapping of pachytene chromosomes in the human male have been obtained with the use of an improved preparative method which incorporates hypotonic pretreatment in 0.125 m KC1 for 60 min at 37°C and the substitution of air-drying for squashing. These preparations meet the minimal requirements we propose: (1) accurately countable pachytene complements are present, and (2) patterns of discrete chromomeres are present late in pachytene. A further observation is that centromere positions of the autosomes are marked early in the pachytene stage by heavily compacted chromomeres; arm ratios derived on the basis of this assumption agree well with those of somatic metaphase chromosomes. The XY bivalent has not exhibited a comparable chromomere pattern, possibly because of its precocious compaction. Pachytene chromomere maps of normal bivalents provide a basis for higher resolution in the analysis of structural variation in the human chromosome complement.