Publisher Summary This chapter describes the use of the microinjection technique as a way to deliver immunopurified antibodies into the cytoplasm of cells in culture. The success in the microinjection of antibodies for neutralization purposes relies mainly on the quality of the antibodies used. Polyclonal antibodies have the advantage of recognizing different epitopes on a same target protein and therefore should have stronger neutralizing activities. By using glass needles, miscellaneous molecules like DNA, RNA, and protein can be injected into the cytoplasm or into the nucleus of cells in culture. This procedure, therefore, allows the investigation of the biological role of different molecules directly in their natural cellular environment. Intracellular microinjection of purified antibodies against different target proteins has been successfully used to inhibit the function of these proteins in vivo. Using this technique, useful information has been obtained about the role of several proteins in different biological situations. This approach can be used to neutralize the regulatory activities of the Fos and Jun proteins in an effort to elucidate the role of these transcription factors in the proliferation of fibroblasts. The requirement of Fos and Jun proteins for cell cycle progression during different growth conditions by monitoring DNA synthesis of the injected cells has been investigated.