Lupins (Lupinus angustifolius L. ev. Yandee), grown in solution culture, have been used to study the sites and process of infection by Bradyrhizobium sp. (Lupinus) and the impairment of nodulation by iron deficiency. Infection leading to nodulation occurred in an area of epidermal cells either lacking root hairs or with very young root hairs at the time of inoculation. Cells aged 13 h or over appeared not to be infected. Infection was initiated in the outer cortex. Rare, short infection threads were evident on day 4 after inoculation, 2 d after the initial division of cortical cells resulting from the bradyrhizobial inoculation. Bacteria had been released into the cytoplasm of cortical cells within 5 d after inoculation. Bacteroids multiplied in the cytoplasm, segregated passively and spread in the infection zones by repeated division of the invaded cells. Under iron deficiency, initial cell division occurred in the outer cortex of host roots, as in iron-sufficient plants after inoculation. Iron deficiency then limited further division of cortical cells. Only a few surviving infection sites developed nodules with normal structure but development was much slower than in iron-sufficient plants.