Immunity against ghosts and exclusion of superinfecting phage were studied in cells infected with T4 phage carrying mutations in the imm (immunity), gene, the s (spackle) gene or in both. There was a strict qualitative parallelism in the development of immunity and exclusion as a function of various factors in cells receiving a specific mutant. Both imm − and s − T4 induced levels of immunity or exclusion close to 90% but the time pattern of appearance of these functions differed with the mutant. Cells receiving the double mutant imm −- s − developed very low resistance to superinfection. In all cases studied, ghost action or superinfecting phage expression followed one-hit kinetics. For a given m.o.i. of superinfection, cells infected with any of these mutants showed lower resistance (2–4×) to phage than to ghosts. There was a good correlation between levels of hydrolysis of the DNA of the superinfecting phage and those of exclusion of a genetic marker, though the amounts of marked DNA hydrolysed did not exceed 50% of the total radioactivity adsorbed. Experiments involving detachment of superinfecting phage envelopes by blending indicated preferential detachment of DNA full envelopes when the primary infecting phage contained the s − mutation. Superinfecting ghosts inhibited formation of infective centers less than protein synthesis in cells receiving s − T4. Experiments to test the exclusion of two genetic markers indicated that in cells infected with imm −, s −, or wt, expression of superinfecting genetic markers occurred through whole genome injection. The genes imm and s were not implicated in the resistance to lauryl sulphate which developed after phage infection.