I. Role of the Natural Immune System in the Control of Tumor Cell Growth.- 1 Role of the Natural Immune System in Control of Primary Tumors and Metastasis.- 1. Introduction.- 2. Role of NK Cells in Host Resistance to Primary-Tumor Growth.- 3. NK Cells and Metastatic Growth.- 4. Role of NK Cells in the Antimetastatic Effects of Anticoagulant Drugs.- 5. Major Histocompatibility Complex (MHC) Antigen Expression and NK Sensitivity of Tumor Cells.- 6. Role in Immune Surveillance against Tumors.- 7. Stimulation of Natural Cell-Mediated Immunity and Treatment of Tumor Metastases.- References.- 2 Lymphokine-Activated Killer Cells: Biology and Therapeutic Efficacy.- 1. Introduction.- 2. Characterization and in Vitro Activity of Murine Lymphokine-Activated Killer Cells.- 3. Adoptive Immunotherapy in Murine Tumor Models.- 4. Mechanisms of Adoptive Immunotherapy and Toxicity in the Murine Model.- 5. Characterization of Human Lymphokine-Activated Killer Cells.- 6. Clinical Trials of Lymphokine-Activated Killer Cells and IL-2 in the Therapy of Metastatic Cancer.- 7. Future Prospects and Conclusions.- References.- Summary of Part I.- II. Involvement of the NIS in the Control of Microbial Iinfections.- 3 Natural Effector Cells in Influenza Virus Infection.- 1. Historical Perspective of NK Cells and Influenza Infection.- 2. Interferon and Influenza Infection.- 3. Role of NK Cells during Influenza Virus Infection.- 4. Pathological Consequences of NK Activity.- 5. Conclusions.- References.- 4 Natural Host Defense Systems Active against Herpes Simplex Virus Infections.- 1. Introduction.- 2. Mouse Studies.- 3. Human Studies.- 4. Conclusions.- References.- 5 Factors Influencing the Control of Virus Infections by Natural Killer Cells.- 1. Introduction.- 2. NK-Resistant and NK-Sensitive Viruses.- 3. Age-Dependent Resistance.- 4. Site of Virus Replication.- 5. Interferon-Mediated Effects on Target Cells.- 6. Mechanisms of Selective Lysis of Virus-Infected Target Cells by NK Cells.- 7. Use olӞ