The Xenopus laevis H1M protein (formerly called B4) is a maternally inherited histone H1 subtype restricted in its expression to early development. Levels of H1M, as well as of the somatic histones H1A, H1B, and H1C, were determined during early embryogenesis using subtype-specific anti-peptide antisera. H1M accumulates late in oogenesis to a titer of approximately 1 ng/unfertilized egg. Following fertilization, H1M persists at slowly decreasing titers for 3 days of development. In contrast, somatic H1 histones are virtually absent from eggs and cleavage-stage embryos (< 80 pg/egg for H1A, < 2 pg/egg for H1B and H1C). H1M thus represents the predominant histone H1 variant in embryos until the beginning of gastrulation, when the amount of newly synthesized H1A increases beyond the 1 ng level. By in situ immunofluorescence, H1M is detected in association with egg and embryonic chromosomes. When expressed at high levels by transient transfection in a Xenopus cell line, H1M competes for chromatin binding with resident H1A. High-level expression of either H1M or H1A causes aberrant chromatin condensation in the transfected cells.