Effect of zinc ions (Zn2+) pretreatment on a) tumour necrosis factor-α (TNF-α), b) interleukin-6 (IL-6) and c) interleukin-1β (IL-1β) levels in Balb/3T3 cells treated with cobalt nanoparticles (CoNPs) and cobalt ions (Co2+). Balb/3T3 cells were pretreated with 50 μM Zn2+ for four hours, followed by treatment with 100 μM Co2+ and 50 μM CoNPs for 24 hours. Data were expressed as the mean ± standard deviation. *p < 0.05 compared with the control group. †p < 0.05 compared with the Co2+ treated group. ‡p < 0.05 compared with the CoNPs treated group.