The biochemical characteristics of a new human profilin isoform are described. We refer to this recently described isoform as profilin II (isoelectric point 5.9) in comparison to profilin I (pI 8.4). We expressed both isoforms in bacteria and compared their actin-binding properties, binding to poly(L-proline), affinities for phosphatidylinositol 4,5-bisphosphate [PtdIns(4,5)P2], and their effects on nucleotide exchange on actin. Profilin I and profilin II have similar affinities for PtdIns(4,5)P2 and poly(L-proline), and both accelerate nucleotide exchange on monomeric actin to the same extent. However, the affinity of profilin I for monomeric actin is about five times higher than the affinity of profilin II for actin. Potential structural differences of profilin I and profilin II that might explain the difference in actin binding are discussed.