Abstract: Cornell K-strain (B15B15) is hyper and GB1 (B13B13) and GB2 (B6B6) are hypo responder chicken genotypes for iNOS gene expression and activity. Two trials were conducted to compare these lines for immune parameters. For mononuclear phagocytic function quantification, 5 chickens per group were injected i.v. with Black India Ink at a rate of 1 mL/kg body weight (BW) at 4 wk. Blood samples were drawn and optical density measured at 640 nm on plate reader. K-strain was significantly better in clearing carbon (P = 0.04) than the GB1 and GB2 chickens at 10 min post i.v. injection. Lymphoproliferation was measured in vitro by incubating 0.2 x 106 peripheral blood leukocytes with 25 μg Concanavalin A. K-strain chicks had the highest lymphoproliferation index than the GB1 and GB2. On the contrary, K-strain chicks had a weaker PHAP-mediated toe-web swelling response as compared with the GB1 and GB2 chicks. No differences were observed between genotypes for the total or IgG anti-SRBC antibodies in primary or secondary immunization response (one trial). However, K-strain exhibited greater persistence (P < 0.05) of IgM levels than GB2 chicks during the decline phase of the booster response. While K chicks had heavier BW (P < 0.01), they had smallest bursal, thymic and splenic weights (relative to BW) vs. GB1 and GB2 in both trials. These findings show that the iNOS hyper responder K-strain chickens also perform better for monocyte/macrophage, T-lymphocyte and perhaps even B-lymphocyte functions even though their lymphoid organ growth lagged behind the iNOS hyporesponder (GB1 and GB2) genotypes.