Effect of F1 proteinase K-treatment on B cell proliferation. B cells in the presence of macrophages (2 × 105/well) were incubated with medium alone (control, □), 45 μg/ml FI (FI, ) or FI (45 μg/ml) treated with proteinase K (FI-pK, ▪). After culturing for 3 days, [3H]-thymidine uptake (cpm) was measured (see Materials and Methods). Results represent the mean ± s.d. This set of results is representative of three individual experiments. The asterisk indicates a significant difference (P < 0·005) compared with FI-stimulated B-cells.