The amino terminal sequence of sea urchin sperm histone H1 and its phosphorylation by egg cytosol. 1. The amino acid sequence of the first 34 residues of sperm histone H1 (SpH1) from Strongylocentrotus purpuratus shows striking similarity with sequences from three South African species. 2. Five contiguous repeats of the tetrapeptide SPBB (where B is R or K) occur between positions 10 and 29. 3. SpH1 was phosphorylated in vitro using egg cytosol as the source of protein kinase and approximately 4.2 mol phosphate were incorporated per mol H1. 4. Sequences of five phosphopeptides of SpH1 show the egg possesses protein kinase activity capable of phosphorylating multiple seryl residues including SPBB in the NH2-, and BBSP in the COOH-end of the protein.