In the first experiment, 64 pigs were used in a factorial (2 x 4) arrangement within a split-plot design with eight blocks of four littermate pigs as the experimental unit for the environment and pig within litter as the experimental unit for dietary treatment to evaluate the effects of conjugated linoleic acid (CLA) on growth and immune cell phenotype when pigs were located in two distinct health status environments. Diets were formulated to contain CLA at 0, .67, 1.33 or 2% and to exceed NRC (1988) nutrient needs of pigs and were self-fed for 7 wk in three phases. We hypothesized that CLA modifies peripheral lymphocyte phenotype, regardless of the environment. We found that as dietary CLA increases there is a linear increase in percentage of CD8 + peripheral lymphocytes (21.7, 22.3, 28.0, and 32.7%; P < .001). The peripheral CD8+ lymphocyte subpopulation includes four distinct cell subsets: TCRgammadeltaCD8alphaalpha T cells, CD3 --CD8alphaalpha lymphocytes (NK cells) and TCRalphabetaCD8alphabeta T cells, and TCRalphabetaCD4+CD8alphaalpha, and putatively TCRalphabetaCD4 --CD8alphaalpha T cells. To more fully phenotypically characterize the lymphocyte subsets that are most influenced by dietary CLA supplementation and to elucidate the functional implications of their expansion, we conducted the second experiment. A total number of 32 pigs were utilized in a design that from day 0 until the first date of vaccination (d 21) was a randomized complete block with pig within block being the experimental unit for dietary treatment. After d 21, the vaccination treatment was added across blocks and the design became a split-plot; pig within block being the experimental unit for dietary treatment and a block of two littermate pigs being the experimental unit for vaccination treatment. Diets were formulated to contain CLA at 0 or 1.33% and were self-fed for 72 d in four phases. CLA synergized with a Brachyspira hyodysenteriae bacterin vaccine to expand the TCRgammadeltaCD8alphaalpha and NK cell subsets. Dietary CLA alone caused a sustained expansion of TCRalphabetaCD8alphabeta T cells (P < .02) that was related functionally to the increase in granzyme activity in lymphocytes from pigs fed CLA (P < .005) and ontogenically to increased CD8alphabeta+ thymocyte, percentages (P < .05) in pigs fed CLA.