In the vegetarian restaurant where he took his food, Kallenbach would often see a young barrister. This was an Indian lawyer who dressed like an Englishman and had taken up the cause of Indian labourers in South Africa. It was not long before the German engineer and M. K. Gandhi became friends.
They had a great deal in common a deep attraction for simple life and working for the good of their fellow beings. At the time Gandhi was struggling for the rights of Indians and Africans in a land dominated by white men.
The form of resistance that Gandhiji used was unique: satyagraha. He would patiently appeal to the good sense of the whites while also refusing to follow their laws that he regarded evil. He was willing to suffer punishment for breaking these laws but refused to hate the white men.
Kallenbach was attracted by this method. He and Gandhiji worked together for the poorest of the poor. They changed their own life style and honoured every useful work. They said that a lawyer or an engineer was not superior to a cobbler or a scavenger. In fact they went to a Chinese cobbler in Johannesburg and learnt to make footwear. And they undertook to clean their own latrines, something most people would not do in these days.
From Ruskin's book Unto This Last, Kallenbach and Gandhi laid down three principles for themselves: (i) the good of the individual is in the good of all; (ii) all work is noble and all are equal; and (iii) a life of labour is worth living.
In 1903 Gandhi's family came over to South Africa. Though Kallenbach became a dear uncle to his three children, Gandhi would not let him buy costly toys for them. They must not feel that they are different from poor people, he would say.
In 1910 Kallenbach, who was a rich man, donated to Gandhi a thousand acre farm belonging to him near Johannesburg. This was a very great gift indeed and was used to run Gandhi's famous 'Tolstoy Farm' that housed the families of satyagrahis.
With the satyagraha campaign in full swing, Gandhi would often go to prison. During such times, Kallenbach would take up the work of editing Indian Opinion, a weekly paper started by Gandhi. Being white, he could not be punished under the South African laws. This angered the white rulers no end, but Kallenbach carried on as a co-worker with Gandhi.
When Gandhi started the Phoenix Ashram near Durban, living as a farmer and labourer, Kallenbach gladly joined in this new life. He built the simple sheds for the inmates, working as a mason and carpenter.
In 1915 Gandhi returned to India. The First World War had broken out. England and Germany were at war with each other. Being a German, Kallenbach was refused entry into India and had to return sadly to South Africa, where he continued his work as a satyagrahi.
Kallenbach did come to India in 1936, when he visited Gandhi's ashram at Sevagram near Wardha. He was ill at that time. Gandhi nursed him back to health himself.  In 1937 the Second World War broke out, Kallenbach was again put into jail by the South African Government. When Kallenbach died of illness a little after this, in 1938, Gandhiji felt he had indeed lost a brother.