Five deworming trials were conducted at the Eastern Research Station located at Haskell, OK during 1992 through 1996. Crossbred cows and their Charolais sired calves were  blocked by sex of calf, calf age and cow age then randomly allotted to  three treatments:  1) non-dewormed control, 2) deworm calf only; 3) deworm cow only; and 4) deworm cow and calf.  Two or three treatments were applied each year including one control group.  Each treatment was applied two or three years. Cows and calves were individually identified and weighed in early June.  Treated animals received label-recommended dosages of ivermectin pour-on.  Pairs grazed in rotation seven bermudagrass pastures overseeded with clover at a stocking rate of  2 acres per cow-calf pair during the 144 to 181-day trials. Initial studies indicated that a low worm infection rate was present in 1991 and 1992.  At that time fecal egg counts ranged from 0 to 28 eggs per 3 gram sample of feces. Deworming cows in late spring had no significant effect on cow summer weight gains up until calf weaning time.  Treating cows but not their calves resulted in  a small advantage in average daily calf weight gains   (0.1 pound/day; P=.1), while treated spring-born calves had significantly greater daily weight gains (0.14 pound/day; P<.04) while nursing non-treated cows. In other words, just deworming the calves resulted in a 21 pound weaning weight advantage over non-treated controls.  Treated calves nursing treated cows had significantly greater average daily weight gains (0.17 pound/day; P<.003) than the untreated calves nursing untreated cows.  Over the approximate 150 day period this weight gain advantage would total about 25 pounds additional weaning weight to calves in this treatment group.  Deworming spring born nursing calves in early summer resulted in significantly greater summer weight gains.