One of the gap class of segmentation genes, Krüppel (Kr), is required for normal thorax and abdominal development of the Drosophila embryo. Its gene product, a zinc-finger type protein, forms a bell-shape concentration gradient in a central position of the blastoderm. Genetic and molecular studies suggested that the Kr protein (KR) may act both as a positive and as a negative regulator of transcription on several other genes of the zygotic segmentation hierarchy. We have examined the regulatory potential of Kr by a series of cotransfection experiments in the Drosophila Schneider cell line system. Different doses of Kr expression plasmid were tested for their ability to drive reporter gene expression mediated by a single 11-base pair KR in vitro binding site common to several putative Kr target genes. Our results show that low amounts of Kr expression plasmid lead to transcriptional activation, whereas high amounts result in repression. Distinct portions of KR other than the DNA-binding domain are required for gene activation and repression, suggesting that KR itself can act as a concentration-dependent positive and negative regulator of transcription.