It has been conclusively shown that immunization of female mice with spermatozoa can significantly decrease their fertility (McLaren, 1964(McLaren, , 1966Edwards, 1964;Bell, 1969;Bell & McLaren, 1970). All of the work by McLaren and Bell (McLaren, 1964, 1966;Bell, 1969;Bell & McLaren, 1970) (McLaren, 1964(McLaren, , 1966Bell, 1969;Bell & McLaren, 1970). However, none of the female mice in Edwards experiments received fewer than 30 \ m=x\106 spermatozoa. This paper reports significant decreases in the fertility of female mice immunized with 15 IO6 spermatozoa by utilization of the carrier effect ('coopera tivi ty').Random-bred Swiss Webster mice were mated before sensitization and only fertile pairs were used in the experiment. A patch, 5 to 6 cm2, was shaved on the belly of the females and 0-05 ml of 0-5% l-fluoro-2-4-dinitrobenzene (FDNB) in a mixture of acetone and olive oil was applied three times at 10-day intervals by painting on the skin. This quantity and dosage schedule had been found to give maximal delayed hypersensitivity to dinitrophenol (DNP) in the CBA strain of mice (Taylor & Iverson, 1971