This paper estimates the costs and returns of on-the-job training (OJT) and its implications for labor-force behavior. It argues that OJT is a significant component of human capital investment, alongside formal education. The study uses data from the 1939, 1949, and 1958 U.S. censuses to estimate the costs of OJT and compare them with the costs of formal schooling. It finds that OJT costs are generally higher than the costs of schooling, but the returns on OJT are comparable to those of formal education. The paper also discusses the relationship between OJT and income and employment differentials among subgroups of the population, such as those classified by education, occupation, sex, and race. It concludes that OJT is an important factor in labor-force behavior, and that the returns on OJT are at least as significant as those on formal education. The study also notes that the returns on OJT may be underestimated due to biases in the data and the difficulty of measuring the true costs of OJT. The paper suggests that further research is needed to better understand the relationship between OJT and labor-force behavior.This paper estimates the costs and returns of on-the-job training (OJT) and its implications for labor-force behavior. It argues that OJT is a significant component of human capital investment, alongside formal education. The study uses data from the 1939, 1949, and 1958 U.S. censuses to estimate the costs of OJT and compare them with the costs of formal schooling. It finds that OJT costs are generally higher than the costs of schooling, but the returns on OJT are comparable to those of formal education. The paper also discusses the relationship between OJT and income and employment differentials among subgroups of the population, such as those classified by education, occupation, sex, and race. It concludes that OJT is an important factor in labor-force behavior, and that the returns on OJT are at least as significant as those on formal education. The study also notes that the returns on OJT may be underestimated due to biases in the data and the difficulty of measuring the true costs of OJT. The paper suggests that further research is needed to better understand the relationship between OJT and labor-force behavior.