High feed costs in pork industry may lead to lower returns. To improve the gains, production cost must be kept minimally. Residual feed intake (RFI) is defined as the difference between observed feed intake and expected feed intake based on estimated maintenance and production requirements. Selection for low RFI is a solution to rising feed costs in pig production. The overall objective of this thesis was to evaluate the effect of selection for RFI on blood cell profile of Yorkshire pigs. In order to do this, an experiment was conducted comparing differences in number of blood cells of Yorkshire pigs that were divergently selected for RFI; estimate the residual correlations between RFI and complete blood count (CBC>/bold) traits, and the heritability estimates of CBC traits. A total of 518 Yorkshire pigs (224 from the high RFI line, HRFI>/bold, and 274 from the low RFI line, LRFI>/bold) were bled between 5 and 6 weeks of age in order to obtain CBCs on them. The HRFI line had significantly higher number of monocytes, lymphocytes, and basophils compared to the LRFI line, while the LRFI line had higher hemoglobin concentration and red blood cell volume (P <0.002). No significant correlations were found between CBC's and RFI across and within the lines (P > 0.05). Of the 15 CBC traits that were measured, three were highly heritable (h2 ranged from 0.56 - 0.62, nine were moderately heritable (0.12 < h2 < 0.47), and three were low heritable (h2 < 0.12). Since most CBC levels are moderate to highly heritable, our results suggest that there is a fairy large genetic component for CBC's and that selection based on CBC traits can be effective. Our results also show that selection for RFI has impacted the number of circulating blood cells. In this experiment we studied only healthy animals not under known pathogen challenge; therefore, our results cannot be directly applied to such a disease challenge situation. Future direction will be to challenge the animals and determine the effect of a challenge on CBC levels