Ultrastructural studies showed that human foetal and neonatal hyaline cartilage contained at least two types of chondrocytes. The predominant chondrocytes of chief cells were characterised by scanty, flat endoplasmic reticulum, few mitochondria, a rudimentary Golgi zone, and by the presence of clear cytoplasmic vacuoles which contained electron-dense granules and beaded filaments. The second type of cells had dark or electron-dense cytoplasm which contained numerous dilated endoplasmic reticulum and prominent Golgi zones; these cells gave rise to slender cytoplasmic processes from which the matrix vesicles were probably derived. The lacunar matrices surrounding both cell types were morphologically different reflecting some functional differences between these cells. Both types of cells participated in the formation of costochondral growth zones. Condrocytes with morphology of the chief cells proliferated, columnised, underwent hypertrophy, and differentiated either into a compact hypertrophic cell or a swollen pale cell. The cells also became hypertrophic and gave rise to increasing numbers of dense vesicles. Calcification was first noted in the longitudinal septa inside or adjacent to matrix vesicles. The recognition of the heterogeneity of chondrocytes in human hyaline cartilage may be helpful in understanding the pathology of the chondrodysplasias.