This paper provides non-experimental field evidence on positive and negative worker reciprocity. We analyze the performance reactions of professional workers to fair and unfair wage allocations in their natural environment. The objects of interest are professional soccer players in the German Bundesliga. This environment enables us to circumvent the main problems of observational studies on reciprocity because there is substantial transparency in individual player values and performance. Our main finding is that workers exhibit both positive and negative reciprocity toward employers who deviate from a player’s perception of a fair market wage. This perception of a fair wage follows from a Mincer-type wage equation that incorporates a worker’s past performance. The different results between changing and non-changing players are in line with theories of fairness perception but cannot be explained by private information from the employers or the personal career concerns of the players. Altogether, our findings provide strong evidence for the external validity of previous laboratory results on gift exchange in the labor market.