A plasmid vector, pJBS633, that facilitates the construction of translational fusions of genes of interest to the coding region of the mature form of TEM beta-lactamase has been developed. Transformants containing in-frame fusions can be identified by their ability to grow when plated at high inocula on agar containing ampicillin (Ap). The cellular location of the beta-lactamase moiety of the fusion proteins can then be determined since only those that direct the translocation of the beta-lactamase across the cytoplasmic membrane to the periplasm result in the ability of individual cells of Escherichia coli to form isolated colonies in the presence of Ap. Conversely, those fusion proteins in which the beta-lactamase moiety remains cytoplasmic do not protect individual cells against Ap. Transformants expressing the latter class of fusion proteins can, however, be identified when plated at high inocula since, as cells start to lyse, the cytoplasmic beta-lactamase activity is released and provides Ap resistance to the surrounding cells. The vector contains the origin of replication of f1 phage so that single-stranded plasmid DNA can be obtained in the appropriate orientation to allow sequencing across the fusion junction using a universal primer complementary to the start of the coding region of mature TEM beta-lactamase. pJBS633 should be useful as a general vector for the construction of beta-lactamase fusions and, in particular, for the analysis of protein export signals and the determination of the organisation of proteins in the E. coli cytoplasmic membrane.