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The direct comparison of in vitro and in vivo results is always impeded by different observation periods (16 h in vitro versus 3 or even 21 days in vivo), different dose rates (kaolin agglomerated in F-12K medium and settled completely), and by the fact that the lung is a dynamic and open system into which cell populations such as monocytes and PMN may invade from the blood stream. Nevertheless, the pronounced cytotoxic effects found for bentonite in vitro seem to be reflected by the elevated concentrations of total protein (2.8-fold of control) and of fibronectin in BALF (6.8-fold of control) on day 3. These proteins may originate from the damaged or leaky lung epithelia, as suggested by histology (see Figure 6g), but also from deteriorated macrophages, which, in a compromised lung, are known to contain fibronectin [35]. Quartz DQ12 elicited less pronounced increases in total protein and fibronectin (1.7- and 3.5-fold of control, respectively) and this difference appears in accordance with its lower bio-activity in vitro. In addition, the pro-inflammatory effect of bentonite indicated by high PMN counts was 3.1-fold larger than the quartz DQ12-induced recruitment of PMN on day 3, possibly reflecting the discrepancy in TNFα expression in vitro, although PMN invasion appears to involve several mediators [36]. Interestingly, the in vitro assays partly overestimated the effects of kaolin in the lung, especially when compared to the effects of quartz DQ12: While all in vitro dose–response curves of quartz DQ12 and kaolin were largely congruent, in vivo only elevated AM counts, and the concentration of fibronectin was nearly identical, whereas PMN counts and the total protein concentration were far lower and hardly different from the vehicle control. Considering the histological pictures of the lung (Figure 5c and Figure 6e), it appears that kaolin does not compromise the lung epithelium known to be involved in the recruitment of PMN from the blood [37]. Overall, the in vitro in vivo comparison shows that the alveolar macrophage assay is a valuable tool to predict early effects of (nano)particles on the alveolar macrophage population but has its limitations if complex biological processes are to be predicted.