Effect of silk lutein extract and marigold lutein extract on NK cell activity. Splenic cells were isolated from BALB/c mice fed lutein extracts from silk cocoons or marigolds daily for 2 weeks and cultured with YAC-1 cell line at a ratio of 100: 1. After 20 hours in a 37°C and 5% CO2 incubator, activity of NK cells was determined by MTT assay. Values represent means ± SEM. ** P < 0.01 compared to the control. SLT10, SLT20; silk lutein extract 10 and 20 mg/kg groups, CLT10, CLT20; marigold lutein extract 10 and 20 mg/kg groups.