Ejaculates and blood plasma were sampled from cocks of three laying lines: Barred Plymouth Rock (BPR), Sussex Light (SU), and Rhode Island Red (RIR). Ejaculates and blood plasma were sampled four times during the laying period of hens. The following ejaculate parameters were determined: sperm motility, concentrations of sperm cells, ejaculate volume. Sperm morphology was examined. In the blood samples, concentrations of glucose, cholesterol, creatinine, calcium, phosphorus, and magnesium were analysed. The motility of spermatozoa of the cocks was 47.0% (BPR), 47.7% (RIR), and 48.3% (SU), respectively. The highest volume of ejaculate was found in BPR line (0.66 cm(3)), the lowest one in SU line (0.46 cm(3), P <= 0.01); the highest sperm concentration was in SU line (2.46 x 10(6)/mm(3)), the lowest one in RIR line (1.96 x 10(6)/mm(3), P <= 0.01). The number of morphologically abnormal sperm cells was similar in all lines -47.0% BPR, 47.7% RIR, and 48.3% SU, respectively. In general, the occurrence of defective spermatozoa was high in all three lines; the most frequent were tail defects (from 20.3 to 29.7%), while sperm cells with developmental anomalies were less frequent (only 0.3 to 0.4%). Calculated phenotypic correlation between sperm motility on the one hand and the occurrence of defective sperm cells on the other was negative (r(p) = -0.28, P <= 0.01), as well as the correlation between sperm motility and sperm numbers, and between sperm motility and ejaculate volume (r(p) = -0.28, P <= 0.01 and r(p) = -0.31, P <= 0.01, respectively). Negative correlations were found between the level of magnesium in blood plasma and numbers of morphologically defective spermatozoa in the ejaculate, defective heads and defective connecting pieces (r(p) = -0.33, P <= 0.01; r(p) = -0.23, P <= 0.05; and r(p) = -0.26, P <= 0.05). Level of magnesium was positively correlated to sperm motility (r(p) = 0.26, P <= 0.05). However, positive correlations existed between concentration of glucose in blood plasma of cocks and numbers of morphologically defective spermatozoa in sampled ejaculates (r(p) = 0.27, P <= 0.01). Our finding is in accordance with the results found in mammals and other animals, and it shows an important role of magnesium as a key contributor to the quality of ejaculate in aviary species, in our case in laying lines of domestic fowl (Gallus domesticus).