RESUMEN
1. Two experiments were conducted to examine the Heterophil/Lymphocyte ratio (H/L) as criterion for selection for resistance to heat stress and some aspects of general resistance in domestic fowls. 2. The first experiment on a small sample (n = 64) of light breed Iraqi local fowls showed that this ratio could be used as an indicator of heat stress resistance. 3. The second experiment on a large sample (n = 1160) of heavy parent stock confirmed the results. It showed that there was a different H/L ratio for the two strains and that there was much individual variability in the H/L ratio within-strain that could be utilised for individual selection for heat resistance. 4. The H/L ratio was highly heritable; Resistant and Sensitive groups, defining all individuals with an H/L ratio over the upper 95% confidence limit as 'Sensitive' and those below as 'Resistant' produced progeny with significantly different H/L ratios. 5. There were indications of differences in mortality between the progeny of the Resistant and the Sensitive groups. 6. The H/L indicator could be used as a criterion to select for heat stress resistance.