Calcium modulation of keratinocyte growth in culture was studied by both transmission (TEM) and scanning electron microscopy (SEM). Under standard culture conditions (1.2-1.8 mM calcium), cells were connected by desmosomes and stratified to 4-6 cell layers. Many aspects of in vitro epidermal maturation were analogous to the in vivo process, with formation of keratohyalin granules, loss of nuclei, formation of cornified envelopes and shedding of cornified cells containing keratin filaments. When the medium calcium concentration was lowered to 0.02-0.1 mM, the pattern of keratinocyte growth was strikingly changed. Cells grew as a monolayer with no desmosomal connections and proliferated rapidly, shedding largely non-cornified cells into the medium. Large bundles of keratin filaments were concentrated in the perinuclear cytoplasm. The elevation of extracellular calcium to 1.2 mM induced low calcium keratinocytes to stratify, keratinize and cornify in a manner analogous to that seen when plated in standard calcium medium. The earliest calcium-induced ultrastructural change was the asymmetric formation of desmosomes between adjacent cells. Desmosomal plaques with associated tonofilaments were observed 5 min after calcium addition; symmetric desmosomes were formed within 1-2 h. This system is presented as a useful model for the study of the regulation of desmosome assembly and disassembly.