The aim of this research was to investigate the relationship between stress, hardiness,: (Challenge, Commitment and Control) as a resistance reso9rce against stress and immune system. Two groups of male and female medical students of Shahid Beheshti and Tehran Universities constituted the sample of this research. One group had a stressful final exam, but the other one didn’t have it. Subjects who suffered from disorders that affected the immune system were excluded. Eventually, the exam group consisted of 147 and the non-exam group of 155 subjects. The level of their hardiness was measured by Personal View Survey (Maddi, 1992). Then 10 subjects with high hardiness and 10 with low hardiness were selected randomly from each group. Peripheral blood was drawn just before the exam. For the non-exam group, blood was drawn at the same time. Number of T-helper cells (CD4), T-Suppressor Cytotoxic cells (CD8), Immunoglobulin M (1gM) and, Cortisol were measured. The results revealed that, there was a positive relationship between hardiness and and 1gM. Also commitment proved to have a positive relationship with CD4 and CD4• There was a positive relationship between control and Furthermore, there was a significant difference between the exam & non-exam groups in Cortisol Level. Moreover, there was a significant difference between the high and low hardiness subjects in CD4 and 1gM. The interaction of hardiness and stress was significant for 1gM and Cortisol. Hardiness served as a source of resistance in the relationship between stress, 1gM and Cortisol. Commitment had a more significant role in hardiness than challenge and control.