Sera from one never-exposed control, two grade 3 and one grade 1/ byssinotic Danish cotton workers were mixed with one strain of B. cereus and two strains of B. pumilus isolated from the air of Danish cotton mills and inoculated onto Vero cells. B. cereus extract was inhibited by all the sera at 48 hours; B. pumilus (D6) extract caused rapid contraction of all the cell sheets within 4 hours at the 1:64 dilution. Only foetal calf serum (used as a control serum) did not prevent the development of cytopathic effects at the 1:4 dilutions at 4 hours. More variable effects were observed with B. pumilus strain D3. At 24 hours typical lace-like effects were observed in the wells containing the extract of B. pumilus (D3) and the highest dilutions of both Grade 3 byssinotic sera.