Much of current biomedical research requires that the expression of a gene (or DNA sequence) under investigation be detected by rapid and reliable means. To achieve this, it is frequently convenient to place a marker (or surrogate) gene under the genetic control of the regulatory sequences of interest. Expression of the marker gene is then monitored by analyzing accumulation of the encoded protein and this, in turn, serves as a measure of gene expression. Here, we describe two methodologies that utilize human placental alkaline phosphatase (hPLAP) as a marker gene. The first exploits a secreted version of alkaline phosphatase (SEAP) to measure gene expression in transfected cells (1), whereas the second uses the naturally occurring membrane-bound form of the protein as a marker for retroviral infection in either tissue culture (2) or challenged animals (3,4). In all cases, the advantages of using hPLAP include the rapidity of the detection procedure, its avoidance of radioisotopes, the relatively low cost of the reagents, the limited number of tissues and cell lines in which hPLAP is ordinarily expressed, and its high temperature stability (the other isozymes of alkaline phosphatase are relatively heat labile).