Spare human oocytes and pre-embryos from an in-vitro fertilization (IVF) programme were individually analysed for ATP and ADP content using a bioluminescence method employing the firefly luciferin-luciferase reaction. The ATP content of oocytes that failed to fertilize in vitro was 1.71 +/- 0.28 pmol (n = 10), pronuclear stage ova 1.93 +/- 0.08 (n = 6), 2-cell stage 1.78 +/- 0.20 (n = 7), 4-cell stage 1.73 +/- 0.15 (n = 6), 6-8 cell stage 2.76 +/- 0.53 (n = 8), morula stage 2.36 +/- 0.68 (n = 8), early blastocyst stage 2.08 +/- 0.25 (n = 7) and expanded blastocyst stage 2.26 +/- 0.15 (n = 3) pmol. The ADP content of these pre-embryos was low in all stages with a small increment in the 2-4 cell stages and the blastocyst stage. This gave an elevated ATP/ADP ratio (range 19-92) indicating a good energy status. The sensitive luciferin-luciferase assay may be a tool for studying the energy status of spare oocytes and pre-embryos under different incubation conditions in human IVF programmes.