Blastogenic activity, eosinophil and total white blood cell counts (TWBC) were examined over a period of 14 weeks in Romney lambs, genetically resistant or susceptible to gastrointestinal nematodes. The lambs were infected with 5000 infective Trichostrongylus colubriformis larvae twice weekly. Compared to preinfection levels, the blastogenic activity of unstimulated lymphocytes in lambs of both lines peaked at week 3, and was significantly higher in resistant than in susceptible lambs. These changes may have been due to in vivo polyclonal activation. Lymphocytes from susceptible sheep responded more strongly to Con A, PHA and PWM than cells from resistant sheep. Counts per minute (c.p.m) for Con A- and PHA-stimulated lymphocytes increased in both lines of sheep from week 2 to week 7 and then returned to initial levels. An increase in c.p.m. in PWM-stimulated cell cultures was observed from weeks 3 to 5 in both groups. The blastogenic activity for LPS-stimulated cultures was significantly higher for resistant than susceptible sheep at weeks 3 and 4. No significant correlations between the decline in faecal egg counts (FEC) and the blastogenic activity was observed. Eosinophil counts in peripheral blood began to increase one week earlier in resistant than in susceptible sheep. No significant correlation between FEC and eosinophil counts was observed in resistant lambs, whereas in susceptible lambs a significant correlation was found between FEC and eosinophil counts at some sampling times. TWBC in resistant lambs steadily increased with infections whereas susceptible lambs showed a decrease until week 5 and then steadily increased. There was no significant correlation between the decline in FEC and TWBC.(ABSTRACT TRUNCATED AT 250 WORDS)