Nuclear factor kappa B (NF-kappa B) has been shown to be an important transcriptional regulatory protein in multiple cell types in response to a number of physiological signals. In lymphocytes it has been implicated in transcriptional regulation of the kappa light chain, MHC, IL-6, and IL-2 receptor genes, depending on the differentiation state of the cell. In the present study we demonstrate that platelet-activating factor (PAF), a phospholipid molecule, activates NF-kappa B and increases kappa light chain mRNA in a human B cell line. Treatment of Ramos cells with PAF (10(-9) to 10(-6) M) increased RNA levels of the NF-kappa B p50 precursor, known as p105, in a dose-dependent manner. p105 RNA levels increased to a maximum observed at 8-10 hr and then diminished by 24 hr; this induction was not blocked by cycloheximide (CHX). PAF induced nuclear kappa B binding at similar concentrations, but more rapidly, attaining maximal levels within 15 min. CHX did not block this activity either. PAF treatment of Ramos cells also resulted in increased levels of RNA for kappa light chain. These results suggest that PAF activates NF-kappa B by at least two mechanisms in these cells, one at the level of post-translational protein activation and the other by increasing the level of RNA for NF-kappa B p105.