Poly-L-lysine hydrobromide is a positively charged synthetic polyamino acid having one HBr per lysine unit. It is a crystalline solid soluble in water. Applications for poly-L-lysine hydrobromide include the promotion of cell adhesion to solid substrates for culture dishes or slides, the conjugation to active molecules for improved activities, the layer-by-layer deposition techniques, and the complexation with nucleic acids for gene expression.