The aim of this study was to compare the sperm quality, levels of zinc, copper, iron, calcium, and magnesium, and the levels of oxidative enzymes catalase, superoxide dismutase, and glutathione peroxidase in seminal plasma and blood serum of a sub-fertile bull with those of seven healthy fertile bulls. Semen was collected using an artificial vagina and frozen using a one-step dilution method. Blood samples were taken from the caudal vein. Sperm viability and morphology were examined using eosin-nigrosin staining, and sperm motility was assessed using CASA. Element levels were measured by atomic absorption spectroscopy, and enzyme levels were determined by ELISA. The results showed that all sperm quality indices (except for ejaculate volume and percentage of cytoplasmic droplets) were significantly lower in the sub-fertile bull compared to the healthy fertile bulls. Comparison of the seminal plasma and blood serum of this bull with those of the healthy bulls revealed that only the magnesium level in the seminal plasma was significantly lower, with no significant differences in other parameters. These findings suggest that measuring the levels of the studied elements and enzymes in blood serum and seminal plasma may not be reliable indicators of reproductive health in bulls, and that post-thaw progressive sperm motility may remain the most important indicator of reproductive health in bulls.