In the treatment of infertility employing in vitro fertilisation and embryo transfer (IVF-EF), oocytes, spermatozoa and pre-embryos are cultured for 48 hours outside the woman's body before they are introduced into the uterus. In addition to the necessary salts, the media in which this culture takes place, also consists of a source of protein. In order to eliminate the variability of patient sera, a prospective, randomized investigation was performed to elucidate whether a well-defined source of protein such as human serum albumin (hSA-hSA 200 mg/ml, Statens Seruminstitute) can replace patient serum as source of protein in the culture of oocytes, spermatozoa and pre-embryos in IVF-ET treatment. The pregnancy rate per transplantation was increased from 30% in the serum group (21 pregnant out of 69 transplantations) to 39% in the albumin group (26 pregnant out of 66 transplantations) but the difference is not significant. On the other hand, the quality of the pre-embryos as assessed by morphological criteria became significantly better and the implantation rate per transplanted pre-embryo was found to be significantly increased in the albumin group. On the basis of this investigation, hSA is recommended as the source of protein, rather than the patient's own serum in the culture of oocytes, spermatozoa and pre-embryos in IVF-ET treatment.