A genome scan was conducted using 196 microsatellite DNA markers spanning 29 autosomal bovine chromosomes and Warner-Bratzler shear force collected at d 2 and 14 postmortem on steaks from the longissimus muscle of 294 progeny from one Brahman x Hereford bull mated to Bos taurus cows to identify QTL for beef tenderness. One QTL was identified and located 28 cM (95% confidence interval is 17 to 40 cM) from the most centromeric marker on BTA15. The QTL interacted significantly with slaughter group. The difference in shear force of steaks aged 14 d postmortem between progeny with the Brahman paternally inherited allele vs those with Hereford was 1.19 phenotypic standard deviations (explained 26% of phenotypic variance) for one slaughter group and was not significant for three other slaughter groups. Apparently, unknown environmental factors present for three of the four slaughter groups were capable of masking the effect of this QTL. The sensitivity of the QTL effect to environmental factors may complicate utilization of markers for genetic improvement. Future research to elucidate the cause of the QTL x slaughter group interaction may lead to improved strategies for controlling variation in meat tenderness via marker-assisted selection, postmortem processing, or live animal management.