The allocation of students to ability tracks is often based on teacher recommendations. These recommendations tend to be biased in favor of students from higher socioeconomic status (SES) backgrounds. While tracking procedures and criteria have been proposed to play a role herein, empirical research is lacking. Using a survey experiment and information from 221 teachers in 69 Dutch schools, I find that teachers in the same school vary in their interpretation of their school's procedure and (relatedly) their own tracking criteria. Teachers who perceive the school procedure to put more weight on students' home environment, and/or (relatedly) put more weight on this themselves, show a stronger SES bias in track recommendations in the survey experiment.