Effects of abscisic acid (ABA) on the melting point (Tm) of nucleohistones when melted at pH 7.0 in a low ionic buffer and at pH 8.2 in a high ionic buffer were investigated. ABA, when added in vivo, affected the Tm during the first 18 hours of ABA pretreatment neither at pH 7.0 nor at pH 8.2. Added in vitro ABA decreased the Tm of nucleohistones melted at pH 8.2 but had no effect at pH 7.0. Histone synthesis was not affected by ABA during the first 12 hours; after 24 hours a significant increase of synthesis above control was observed. It is concluded that ABA does not affect binding between DNA and proteins in the nucleohistone complex, nor has it a direct effect on histone synthesis.