Lactate dehydrogenase (LDH) has been used extensively as a marker for cell death both in vitro and in vivo. The release of LDH into tissue culture medium accurately reflects cell viability in vitro. We have investigated the relationship between cell concentration and total LDH activity in samples of cell lysate. Although there are differences in the amount of LDH present in different cell types, the total enzyme activity in a sample of cell lysate is directly proportional to the concentration of cells in the sample. The measurement of LDH activity in vitro provides a sensitive, accurate and cost-effective alternative to the use of either radioisotopic or dye-based assays for the determination of cell numbers.