Male pronuclear (MPN) formation in oocytes after in vitro maturation (IVM) was compared with that of matured follicular oocytes that had matured in vivo (controls). Cumulus–oocyte complexes were matured in vitro for 13 h in modified Tyrode's solution (TLP–PVA); cumulus-free oocytes were then incubated in 20% oviductal fluid for 3 h, and washed and capacitated spermatozoa were added. MPN formation was significantly lower (P < 0.05) in IVM oocytes 3 to 12 h after insemination (0 to 34%, respectively) than in control oocytes (range, 98–100%). Female pronuclear formation was 84–100% in controls and IVM oocytes, but spermatozoa incompletely decondensed in IVM oocytes. The addition of 10 μmol l−1 during IVM, significantly increased (P < 0.05) MPN formation (from 17% in the absence of cysteine to 47% in the presence of cysteine), but was lower than that in controls (88%). During IVM, the addition of 10% serum or gonadotrophins (FSH and LH) with or without amino acids did not support MPN formation without cysteamine, whereas the treatment with gonadotrophins and 11 amino acids plus 200 μmol cysteamine l−1 (82%) equalled controls (92%). Development of oocytes after IVM (in 0, 10, 20% serum) in TLP–PVA, gonadotrophins, 11 amino acids and 200 μmol cysteamine l−1 was compared with development in controls. Of the IVM treatments, 20% serum was inferior at fertilization, but yielded the highest percentage of fertilized oocytes developing to or beyond the four-cell stage (20% serum versus controls, respectively): fertilized oocytes, 75% versus 88%; ≥four-cell embryo, 40% versus 53%; blastocyst, 8% versus 14%. It was concluded that during IVM, gonadotrophins plus 11 amino acids interacted with cysteamine, enhancing the decondensation of spermatozoa and MPN formation; oocytes matured in this medium with 20% serum were fertilized and some developed to the blastocyst stage.