A method of microinjection: delivering monoclonal antibody 1223 into sea urchin embryos. In this paper, a simpler method of microinjecting sea urchin embryos without using the conventional microinjection chamber designed by Kiehart is reported. A trough was made on a surface of 0.6% agarose gel dissolved in artificial sea water. Approximately fifty hatched embryos could be loaded in the trough and, consequently, swimming embryos were trapped in the trough. Monoclonal antibody (mAB) 1223 which blocks spiculogenesis in vitro was delivered into the blastocoels of sea urchin embryos to test whether this antibody inhibits spiculogenesis in vivo and also, whether this new technique is effective for the microinjection of the sea urchin embryos. The embryos were injected with mAB1223 at the hatched blastula, early mesenchyme blastula and early gastrula stages, and 63%, 90% and 97% of the embryos did not form spicules at the late gastrula stage, respectively. Therefore, mAB1223 was shown to also block spiculogenesis in vivo. From the fact that spiculogenesis occurred at a lower rate when mAB1223 was injected at the hatched blastula stage than at later stages, it may be speculated that endogenous proteases degraded the injected antibodies. Using this technique, extracellular events in the blastocoel or the function of certain molecules expressed in blastocoel can be easily investigated in vivo.