Colostrum is rich in IGF-I and IGF-II, and the dietary effects of recombinant human (rh)IGF-I on the newborn are of interest. The objective of this study was to examine the effects of dietary rhIGF-I on intestinal tissue growth and populations of IGF receptors. Twenty-three male diary calves were fed one of three experimental diets: 1) milk replacer plus isolated colostrum-derived globulins (MR-), 2) same as 1 plus 750 ng of rhIGF-I/mL (MR+), or 3) pooled cow colostrum (COL). After the first four feedings, all calves received milk replacer without additional globulins; calves fed the MR+ diet continued to receive the addition of 750 ng of rhIGF-I/mL until the experiment ended at 7 d after birth. Calves were killed and intestinal tissue was collected for in vitro [3H]thymidine incorporation studies. Incorporation differed among intestinal regions (duodenum, jejunum, and ileum). The MR+ calves had greater (P < .01) [3H]thymidine incorporation per unit of DNA than either the COL or MR- calves (31.8 vs 18.6 and 11.5 x 10(3) dpm/microgram of DNA, respectively). Competitive binding analysis indicated the presence of specific type 1 and type 2 intestinal IGF receptors. The IGF-I was more potent than IGF-II and insulin at inhibiting [125I]rhIGF-I binding (ED50 was 1.84, 9.17, and 1.91 ng/mL, respectively). The IGF-II was the only ligand capable of inhibiting [125I]rhIGF-II binding (ED50 was .30 nmol/mL).(ABSTRACT TRUNCATED AT 250 WORDS)