This research involved ninety-six S1 maize lines with the opaque-2 gene that carne from populations 63 (26 lines), 65 (12 lines), and 68 (58 lines) from the CIMMYT maize breeding programo The S1 lines selected in the laboratory because of their high lysine content were crossed with five testers that had a different genetic base: a synthetic variety, two early lines derived from this variety, and two single intermediate crosses of normal performance. The test crosses were evaluated and compared with five control s (two commercial and three experimental hybrids) using a randomized complete block design in two sites in Mexico during 1992. The objecti ves of this research were to select SI lines in early generations according to their agronomic performance, grain lysine content, and GCA grain yield, and to compare testers' efficiency in discriminating S1 lines. On average, the SI lines of population 63 showed a greater grain yield as compared to that of populations 65 and 68 combined with all testers, except in the case of the synthetic tester YS- 201-M. A line from population 65 combined with tester B3 x B5 showed the highest grain yield (10,23 t ha-I), representing a superiority of 4% above the best commercial hybrid, AN- 447. A total ofthirty-one Sllines were selected based on their agronomic attributes and grain yield: 13, 14, and 4 from populations 63, 65 and 68 respectively]. Tester B3 x B5 tumed out to be the best line discriminator according to the following criteria: grain yield, percentage of ears damaged by Fusarium spp., and number of rotten ears.