Northern analysis has revealed substantial differences in mRNA accumulation of the two histone H3 gene variants represented by pH3c-1 and pH3c-11 cDNA clones. Both in partially synchronized cell suspension cultures and in protoplast-derived cells from alfalfa, Medicago varia, the maximal level of the histone H3-1 gene transcript coincided with the peak in [(3)H]thymidine incorporation. Histone H3-11 mRNA was detectable in cells throughout the period of the cell cycle studied. Various stress factors such as medium replacement, enzyme digestion of the cell wall, osmotic shock, and auxin treatment considerably increased the level of the histone H3-11 transcript. In alfalfa (Medicago sativa), the presence of H3-11 mRNA in unorganized tissues of microcallus suspension and in somatic embryos induced by auxin treatment supports the idea that this H3 variant exists in a continously active state of transcription. During embryo development, the early globular stage embryos showed increased accumulation of histone H3-11 mRNA in comparison with the later stages. The highest level of the histone H3-1 transcript was detectable 1 day after treatment of callus tissues with 2,4-dichlorophenoxyacetic acid. Somatic embryos contained appreciable levels of histone H3-1 transcripts at all stages of somatic embryo development. These observations suggest that the histone H3-1 gene belòngs to the class of replication-dependent histone genes. The histone H3-11 gene showed characteristics of a constitutively expressed replacement-type histone gene, with a specific characteristic that external factors can influence the level of gene transcription.