A more rapid determination of ploidy level in tetraploid ryegrass (Lolium spp.) cultivars than cytological chromosome counting would be beneficial to the grass seed industry. Our objectives were to develop protocols for using flow cytometry in seed testing and to validate ploidy level by using flow cytometry and root tip chromosome counts. DNA content was determined by flow cytometry using leaf tissue of up to 200 seedlings from seed lots of three tetraploid cultivars each of perennial ryegrass (L. perenne), annual or Italian ryegrass (L. multiflorum) and intermediate ryegrass (L. X hybridum). Chromosome numbers were counted using root tip cells from up to 40 plants in each seed lot. Ploidy determined by flow cytometric comparison of G1 peaks from diploid and tetraploid plants was compared with chromosome counts on the same plants. Flow cytometry easily determined ploidy level and was sensitive enough to detect aneuploid plants. Determining chromosome number from root tip cells was slow and laborious; we were unable to detect minor variation of chromosome numbers and counts could not be accomplished on a sufficient number of plants to accurately detect mixtures in ploidy levels. We recommend the acceptance of flow cytometry as a means for determining ploidy level in commercial ryegrass seed testing because of its accuracy, ease of use and greater throughput than classical cytological approaches.