Abstract The chemotactic activity of chicken blood mononuclear leukocytes was examined in partially-developed 15I 5- B-congenic chicken lines using Enterobacter cloacae culture supernatant and formyl-methionyl-leucyl-phenylanine (f-met-leu-phe) (10 −5 M). Cells from seven different coded B-congenic lines were used to study each chemoattractant in vitro. Mononuclear cells from lines .15I- B 5,.C- B 12 and the background line 15I 5 ( B 15) exhibited a significantly greater directed migration to bacterial supernatant than did cells from four lines carrying the B 2, B 2, B 13, and B 19 haplotypes, respectively. Similarly response to f-met-leu-phe was greatest in lines .15I- B 5, .C- B 12 and .N- B 21, with the same four lines exhibiting a significantly lower response. Since f-met-leu-phe was originally isolated from bacteria, the results indicate that these lines possess differential chemotactic responses to certain bacterially-derived chemoattractants. Major histocompatibility complex (MHC) differences between the lines may serve as a genetic basis for the differential responses. Extrapolation of these results to other chemotactic-receptor systems would require further examination.