A reproducible 1-way mixed lymphocyte culture (MLC) assay was used to study the ontogeny of MLC in N, P, RPRL-72 and RPRL-63 strains of chickens. The chicks were progeny of specific-pathogen-free and lymphoid leukosis virus-free parents and grown in common isolators. When cells were from responder and stimulator chickens of the same age, the RPRL-72 chickens cells responded by 8 wk, whereas cells from chickens of the other 3 strains did not respond significantly until after 14 wk of age. In MLC with 6- and 32-wk-old RPRL-72 and N birds, the age of the responder was not crucial. However, young or old N birds responded extremely well to old 72 stimulator cells, whereas young 72 cells stimulated no, or minimal, response. Thus the age of the stimulator cell is vary important in chicken MLC and appears to depend upon the responder chicken strain.