Human oocytes can be fertilized with high rates of success under in-vitro conditions even if only low numbers of spermatozoa are used. A culture system has been developed in which fertilization is performed in haematocrit capillary tubes (length 75 mm; i.d. 0.8–0.9 mm). Oocytes were fertilized in 5–10μl of different sperm suspensions containing a total of 500, 1000, 2000 and 4000 spermatozoa per oocyte (0.1–0.4 × 106 spermatozoa/ml). Oocytes were obtained from 10 patients participating in an in-vitro fertilization programme; of these, 32 oocytes were fertilized in capillary tubes and 32 oocytes were cultured using standard methods (1 ml culture medium in tissue culture tubes; 0.1–0.2 × 106 spermatozoa/ml). The overall fertilization rate of oocytes cultured in tissue culture tubes was 78% (25/32) and the fertilization rates in capillary tubes using 4000, 2000, 1000 or 500 spermatozoa per oocyte were 71% (5/7), 86% (6/7), 60% (6/10)and 50% (4/8), respectively. The fertilization rate of mature oocytes was higher compared with immature oocytes when fertilization was performed in culture tubes (83 and 63%) or in capillary tubes (74 and 44%). Fertilization in capillary tubes using a 10μl of oocyte and spermatozoa suspension compared to 5μl seemed to provide better culture conditions, resulting in higher fertilization and cleavage rates. These preliminary results indicate that fertilization of human oocytes under in-vitrio conditions can be achieved even with very low numbers of spermatozoa