Two fractions of earthworm humic substances, differing in nominal molecular weight, containing a very low amount of free IAA, and exhibiting auxin-like properties, were prepared and characterised by infrared and 1 H-NMR spectroscopy. In this study we investigate their effects on stomatal opening, as influenced by phospholipase A 2 , in leaf of the Argenteum mutant of pea (Pisum sativum L.) Both of the humic fractions caused stomatal opening in the epidermal peels. The response showed a broad biphasic dose dependence and the effective concentrations were similar for the two fractions. The maximal stomatal apertures in response to both humic substances were similar to that caused by IAA and somewhat less than the response to white light or fusicoccin. Two inhibitors of phospholipase A 2 selectively blocked the response of stomata to both IAA and humic substances, without affecting the response to light or fusicoccin. We conclude that stomatal opening in response to auxin and humic substances involves activation of a phospholipase A 2 that is not involved in signalling the response to light or fusicoccin.