A microfluorimetric assay was designed to monitor beta-glucuronidase (beta-gluc) activity during the early development of mouse embryos and was also applied to measure beta-gluc activity in human sperm cells and single human oocytes. Mouse oocytes and preimplantation embryos at different stages of development, failed fertilized human oocytes, and human sperm cells were collected, and beta-gluc activity was analysed. In the mouse, beta-gluc activity could be measured starting from the morula stage on in a pooled sample of 15 embryos and at blastocyst formation in one single embryo. beta-Gluc activity was low in human sperm cells. The enzyme could be readily demonstrated in 87 per cent of non-fertilized human oocytes. Considerable variation in beta-gluc activity was noticed between individual oocytes.