Bcl-2 protein family members are among the key regulators of the apoptosis effector phase. Therefore, we investigated the ability of synthetic peptides derived from proteins of the Bcl-2 family, namely, the NH₂-terminal region of Bcl-2 (Bcl2_syn), a central domain of Bax (Bax_syn), and a central domain of Bak (Bak_syn) to interfere with the apoptotic process in LLC-PK₁ cells. Apoptosis was induced by tacrolimus or lipopolysaccharide treatment, and microinjection of Bcl2_syn into stimulated LLC-PK₁ cells significantly reduced the percentage of apoptotic cells detected within 4 h after the treatment. Microinjection of Bax_syn or Bax_syn, in contrast, induced apoptosis in otherwise untreated LLC-PK₁ cells during the same period of time. A random sequence control peptide (Control_syn), which served as a negative control, as well as FITC-labeled dextran, which was coinjected in all experiments for visualization, were ineffective in either preventing or inducing apoptosis. These results suggest that synthetic peptides mimicking the functional domains of proteins of the Bcl-2 family are capable of regulating apoptosis when microinjected into LLC-PK₁ cells in vivo. Analogs to these regulatory peptides could therefore provide valuable lead compounds in the therapeutical context.