Several factors exert considerable influence on oocyte production and quality, such as donor age, nutrition, season and others. However, very few studies have been done to verify the effects of breed on the quantity or quality of oocytes, or both. In Brazil, the Nellore breed has been developed as a result of many years of selective breeding from 6 major ancestor bulls imported from India in the 1960s. These animals have characterised their respective lineages, as evidenced by the well-defined genetic traits. This study was carried out to investigate the influence of the genetic lineage of oocyte donors on the number and quality of oocytes obtained through ultrasound-guided follicular aspiration in Nellore cows derived from 2 lineages of bulls [Karvadi (K) and Taj Mahal (T)]. Both maternal (Km and Tm) and paternal (Kp and Tp) lineages were investigated. Oocyte aspirations were repeatedly performed with an aspiration interval of 15 days without the superovulation procedure in 56 donor females over a period of 2 years. At the beginning of the experiment, the animals were on average 78 ± 12 months old with a body condition score of 3.6 ± 0.08 (on a 5-point scale). Recovered cumulus-oocyte complexes were counted, morphologically examined and classified into 7 categories (grades I to VII) according to the number of layers of the cumulus-oocyte and the appearance of the cytoplasm. Data were analysed using ANOVA and Tukey's test after transformation into log(x + 1). Values are reported as means ± s.e.m. and differences (P > 0.05) were taken as significant. The mean number of oocytes retrieved from donors of lineage Tp-Tm was higher (n = 9; 28.23 ± 1.92; P < 0.05) than that obtained for lineages Kp-Tm (n = 11; 21.34 ± 1.32), Kp-Km (n = 15; 21.28 ± 1.73) and Tp-Km (n = 21; 16.72 ± 1.31). There was no significant difference in the mean number of recovered oocytes between donors of lineages Kp-Tm and Kp-Km; however, lineage Tp-Km yielded a lower number of oocytes. Donors of lineage Kp yielded more (P < 0.05) oocytes of grade III than lineage Tp (10.11 ± 0.66 vs 8.79 ± 0.58, respectively). These lineages produced a higher mean number of oocytes graded as I, II and III (3.14 ± 0.21, 4.93 ± 0.33 and 10.11 ± 0.66 vs 3.19 ± 0.21, 5.59 ± 0.44 and 8.79 ± 0.58, respectively) than any other categories. However, more oocytes (P < 0.05) of grades I, II and III were obtained for lineage Tm (3.64 ± 0.25, 5.9 ± 0.42 and 11.67 ± 0.67) than for lineage Km (2.85 ± 0.19, 4.92 ± 0.40 and 7.73 ± 0.56, respectively), with similar results for oocytes of grades IV, V, VI and VII. In conclusion, the genetic lineage of the oocyte donor influenced the production and quality of oocytes obtained through ultrasound-guided follicular aspiration in Nellore cows. Selection of donor cows based on their oocyte morphology and availability can further improve IFV programs. [ABSTRACT FROM AUTHOR]