Follicular fluid, of varying maturity, (day 5 - 16 of cycle) was collected from the largest Graafian follicle of each of 22 ovulatory patients during laparoscopic procedures. Three samples were blood-stained and discarded. The mitogenic potential of each sample was determined using bovine aortic endothelial cells in the CellTiter 96TM Non-Radioactive Cell Proliferation/Cytotoxicity Assay system. Intra- and inter-plate coefficients of variation were <9%. The follicular fluid samples induced cell doubling times which varied from approximately 12-24 h and final cell numbers which, in the individual wells, ranged from 7828-30 900 (starting number 2000/well). Follicular fluid total protein content was unrelated to the mitogenic potential, (R2 = 0%). Serum oestradiol was negatively correlated with the mitogenic potential (R2 = 26%). No correlation was found with day of the menstrual cycle (R2 = 4.3%), maximum follicular diameter (R2 = 1.8%), or serum concentration of progesterone (R2 = 0.7%), luteinizing hormone (LH) (R2 = 1.5%) or follicle stimulating hormone (R2 = 0.1%). Five subjects were in 'early' and six in 'mid'-follicular phase, six were in 'early' and two in 'late' LH surge. There was no difference in the mitogenic response between these four groups by one-way analysis of variance (F = 0.21; P = 0.89). It is concluded that the mitogenic potential of human follicular fluid is not related to Graafian follicle maturity or, more particularly, to the LH surge.