Microsomes from Kluyveromyces marxianus GK1005 examined by carbon monoxide difference spectroscopy showed no evidence of cytochrome P450, in contrast to microsomes isolated from a control strain of Saccharomyces cerevisiae. Benzo[a]pyrene produced a typical Type I-binding spectrum with microsomes of both yeasts, with K values of 82 mu M (S. cerevisiae) and 70 mu M (K. marxianus). While aflatoxin B-1 generated a typical Type I-binding spectrum with microsomes from S. cerevisiae (K-s of 178 mu M), the toxin did not produce a recognisable binding spectrum with microsomes from K. marxianus.