Presuckle colostral samples and lamb serum samples taken 36 h postpartum were assayed for immunoglobulin G1 (IgG1) concentration (mg/ml) using single radial immunodiffusion. Breeds sampled included Polypay (P), Rambouillet (R), Targhee (T), Columbia (C), Finnish Landrace (F) and Finn crosses (Fx). Sources of variation examined in IgG1 concentration in colostrum (dam trait) included dam's sire breed, dam's sire, age of ewe and number of lambs born. All sources of variation were statistically significant. Least-squares means of IgG1 levels for sire breed were 80, 64, 67, 64, 72 and 69 mg/ml for P, R, T, C, F and Fx breed groups, respectively. A fetal stimulus may exist to increase the mass of IgG1 in colostrum available for multiple births (61, 69 and 77 mg/ml for single, twin and triplet, respectively). Ewe age was a significant source of variation because of a high mean concentration of IgG1 in the yearling's colostrum (100 mg/ml), whereas only slight differences occurred between the other age groups (65 to 67 mg/ml), except for the 7-yr older group (53 mg/ml). Sources of variation examined in IgG1 concentration of lamb serum at 36 h postpartum (lamb trait) included lamb's sire breed, lamb's sire, age of dam, birth type and sex, with dam's colostral IgG1 concentration and day born as covariates. Sire within breed, birth type and the two covariates were significant. Least squares means for sire breed were 36, 32, 33, 32, 31 and 32 mg/ml of serum for P, R, T, C, F and Fx groups, respectively. Lamb serum IgG1 decreased as birth type increased. The heritability of IgG1, estimated by paternal half-sib analyses, was .19 +/- .12 for colostrum and .18 +/- .06 for lamb serum.