In this biology activity, learners use plastic pipettes to cut wells into the solid gel layer of agar in petri dishes and place solutions of simulated antigen and antibody preparations into the wells. The antigens and antibodies diffuse into the gel layer and react to form a precipitate. This activity demonstrates the biological phenomenon of the formation of a precipitate when an antigen reacts with an antibody. The exercise can be used to illustrate the specificity of antigen-antibody reactions, showing that a precipitation reaction only occurs when an antibody reacts with the antigen that was used to induce the formation of the antibody. The exercise is also a general demonstration of diffusion. Adult supervision is recommended.