Effects of NP exposures on the ability of NK cells to lyse tumor cells. NK cells were exposed to 1-10 μM NP for 1 hr, 24 hr, 48 hr, or 6 d. Results were from three separate experiments using different donors (triplicate determinations for each experiment, n = 9, mean ± SD), as described in Figure 1. *Statistically significant change as compared to control cells at that same length of incubation (p < 0.05).