Wortmannin (Wm), a steroid-like molecule of 428.4 Da, appears to be unstable in biological fluids (apparent chemical instability), yet it exhibits an antiproliferative activity in assays employing a 48 hr incubation period (prolonged bioactivity), a situation we refer to as the "wortmannin paradox." Under physiological conditions, Wm covalently reacts with nucleophiles such as the side chains of cysteine, N-methyl hexanoic acid, lysine, or proline at the C20 position on the furan ring. Like Wm, WmC20 amino acid derivatives had significant antiproliferative activities. Three Wm derivatives, WmC20-proline, WmC20-cysteine, and a WmC20-N-methyl hexanoic acid, generated Wm that then reacted with lysine in an exchange-type reaction. This unusual, reversible, covalent reaction of Wm with nucleophiles under physiological conditions provides an explanation for the wortmannin paradox.