We describe a series of plasmid vectors which contain modular features particularly useful for studying gene expression in eukaryotic systems. The vectors contain the Escherichia coli beta-galactosidase (beta Gal)-encoding region (the lacZ gene) flanked by unique polylinker segments on the 5' and 3' ends, and several combinations of a variety of modules: a selectable marker (an amber suppressor tRNA), a translational initiation region, a synthetic intron segment, the early polyadenylation signal from SV40, and 3' regions from two nematode genes. A segment encoding the nuclear localization peptide from the SV40 T antigen is incorporated into many of the constructs, leading to beta Gal accumulation in nuclei, which can facilitate identification of producing cells in complex tissues. To make functional beta Gal fusions to secreted proteins, we constructed plasmids with an alternate module encoding a synthetic transmembrane domain upstream from lacZ. This domain is designed to stop transfer of secreted proteins across the membrane during secretion, allowing the beta Gal domain of the fusion polypeptide to remain in the cytoplasm and thus function in enzymatic assays. We have used the vectors to analyze expression of several genes in the nematode Caenorhabditis elegans, and have demonstrated in these studies that lacZ can be expressed in a wide variety of different tissues and cell types. These vectors should be useful in studying gene expression both in C. elegans and in other experimental systems.