Measurements were made on electron micrographs of 6 human anterior pituitary glands of the maximal diameters of secretory granules in 130 cells, average 91 granules per cell. This was done in an attempt to classify the various hormone types solely by differences in the mean diameters of the secretory granules. The range of diameters in nanometers could be segregated into 6 subgroups of the following suggested functions: 135.5 .+-. 13.6 TSH [thyrotropin], 181.6 .+-. 9.3 LH [luteinizing hormone], 226.0 .+-. 19.4 FSH [follicle stimulating hormone], 356.8 .+-. 30.5 ACTH/MSH [melanocyte stimulating hormone], 452.9 .+-. 23.8 GH [growth hormone], 559.9 .+-. 38.0 prolactin.