The use of microinjection to study gene function in zebrafish has become widespread. Its applications include ectopic expression of genes by introducing DNA (1) or RNA into embryos and perturbation of gene function by injecting RNA-encoding truncated proteins (2, 3), blocking antibodies (4), or antisense morpholino oligonucleotides (5). The method involves the injection of DNA, RNA, or morpholino into the yolk or cytoplasm of one-cell-stage embryos using a pressure microinjector and micromanipulator as described here. The ability to transplant cells and tissue allows one to assess the interactions and behaviors of cells and tissues when placed in ectopic locations or different genetic backgrounds. The combination of microinjection with transplantation complements the genetic tools offered by zebrafish, facilitates studies of inductive interactions and cell behaviors, and enables the generation of germ-line chimeras.