A prospective, randomized study was undertaken to compare the use of Earle's balanced salt solution (EBSS) prepared 'in house' with that produced commercially, in 448 cycles of therapeutic in-vitro fertilization. Outcome was assessed in terms of fertilization and cleavage rates, embryo morphology, and implantation rates following embryo transfer. The only differences that were found between the two media in any of the outcome parameters were in the number of cycles with failed fertilization (1/218 in 'in house' medium compared with 10/230 in commercially prepared medium; P = 0.0186), and in the rate at which embryos cleaved. Thus, while the median number of blastomeres per embryo was no different in the two groups at 46-49 h post insemination (three in embryos cultured in 'in-house' medium, compared with four in those cultured in commercially prepared medium; P > 0.1), the number of embryos per cycle that had cleaved to the 4-cell stage by 46-49 h post insemination was significantly greater in the Medi-Cult than in the EBSS medium (P < 0.001).