Abstract Carbon black (CB) is a material produced by the combustion of hydrocarbon gases or liquids and widely used in various industrial settings. The International Agency for Research on Cancer has classified CB as possibly carcinogenic to humans. Previous studies suggest that CB might induce lung inflammation and injury and it is well known that inflammation can play an important role in tumor development. However, the relationship between CB exposure and peripheral white blood cell counts is still not clear. To address this issue, we conducted a cross-sectional molecular epidemiology study of 106 CB-exposed male workers responsible for packing CB in a CB manufacturing facility that produced very high purity CB and 112 unexposed male control workers from the same geographic region who were frequency-matched to the exposed workers by age. The average number of years worked in the factory for the exposed workers was 12.5 years. Peripheral blood samples were collected and a complete blood cell count (CBC) and lymphocyte subset analysis were conducted. There was a borderline significant increase in eosinophil count, and a significant decrease in total lymphocyte and NK cell counts, among the exposed vs. control workers. Among non-smokers (35 exposed and 29 controls), CB exposure was associated with a borderline significant increase in total white blood cell and neutrophil counts, a significant increase in eosinophil counts, and a significant decrease in total lymphocyte, CD3+ T cell, CD8+ T cell and NK cell counts. There were no significant differences between exposed and control workers for any white blood cell measure among smokers. Also, there was some evidence of an interaction between carbon black exposure and smoking status for neutrophil, total lymphocyte, CD3+ T cell, and CD8+ T cell counts. Our results suggest that CB exposure may be associated with altered levels of cells that play an important role in the inflammatory process among non-smokers. Also, there was evidence for an interaction between CB exposure and tobacco use. These findings may provide new insights into particulate-associated disease. Citation Format: Yufei Dai, Rong Zhang, Yong Niu, Huawei Duan, Tao Meng, Meng Ye, Meili Shen, Ping Bin, Shanfa Yu, Roel Vermeulen, Nathaniel Rothman, Qing Lan, Yuxin Zheng. Effects of occupational exposure to carbon black on peripheral white blood cell counts. [abstract]. In: Proceedings of the 106th Annual Meeting of the American Association for Cancer Research; 2015 Apr 18-22; Philadelphia, PA. Philadelphia (PA): AACR; Cancer Res 2015;75(15 Suppl):Abstract nr 839. doi:10.1158/1538-7445.AM2015-839