We wished to assess the contribution of the lens to the maintenance of the high level of lactic acid found in human aqueous humour. The lactic acid in aqueous humour was estimated by Pryce's modification of the Barker-Summerson method in 41 subjects who were divided into four groups: those with early cataract, those with advanced nuclear cataract, those with mature cataract and those with hypermature (morgagnian type) cataract. Statistical evaluation of the results showed that there was no significant difference in the lactic acid level between the four groups. We assume that the lens does not contribute significantly to the high concentration of lactate found in human aqueous humour.