Components with hapten-like properties were isolated from the nondialyzable fraction, i.e. the retentate (R) and the dialyzable fractions of the aqueous extract of Kentucky Blue Grass pollen (KBG aq.ext.), by preparative isoelectrofocussing on Sephadex G-100 gel. These haptenic components could not elicit the passive cutaneous anaphylaxis (PCA) reactions in rats passively sensitized with a murine reaginic antiserum to R, but could inhibit completely and specifically the PCA reaction which is normally elicitable with R. It was concluded that the specificity of the murine IgE antibodies was directed to a determinants) which was common to either allergenic or haptenic fractions. Moreover, by employing a pool of human sera from individuals allergic to KBG pollen in the radio-allergosorbent test procedure, it was apparent that most of the haptenic fractions lacked some of the specificities present on allergenic components of R that were recognized by the human IgE antibodies. Evidence was obtained to suggest that the electrophoretic heterogeneity of allergenic components present in various fractions of KBG aq.ext. may be due primarily to differences in their net charge, rather than to differences in their allergenic specificity.