Equivalent doses of follicle stimulating hormone (FSH) produced the same number of embryos and ova from a single flush irrespective of the luteinising hormone (LH) content of the superovulating drug (P less than 0.108). As the LH content of the FSH increased, the proportion of transferable embryos decreased (P less than 0.001) because the proportion fertilised decreased (P less than 0.001) and the degeneration rate of the fertilised embryos increased (P less than 0.002). FSH-W free from detectable luteinising hormone produced 8.8 embryos per flush of which 5.7 were transferable, representing 7.6 fertilised embryos of which 21 per cent had degenerated. The addition of a very small quantity of LH (FSH/LH ratio more than 500) resulted in 5.8 transferable embryos from a total of 10.6, of which 9.0 had been fertilised and 34 per cent of those fertilised had degenerated. Commercial FSH-P (FSH/LH less than 100) produced 3.3 transferable embryos from a total of 8.1, of which six had been fertilised and 39 per cent of those fertilised had degenerated. The luteinising hormone content of FSH-P has to be controlled and limited for optimum superovulation in cattle.