There was nobody home at the neat brick house at 4124 Jennifer St. in Northwest Washington. The elderly Japanese lady who answered the door at 4122 spoke no English but smiled and bowed repeatedly, clasping her palms together before her face in the ancient gesture of respect.

It was not until he got to the duplex at 4116 that William Middendorf, one of more than 20,000 volunteer Catholic census workers who knocked on doors all over the District and adjacent Maryland counties yesterday, found what he was searching for: a Catholic household. But even there, the woman who answered the knock explained in heavily accented English that the family did not attend Blessed Sacrament, the parish in which they resided, but the Italian parish downtown.

The massive information-gathering project yesterday was part of the effort by Archbishop James A. Hickey, who came to the archdiocese a little more than a year ago, to get more precise information on the Catholics under his charge and their relationship to the church.

The archdiocesan directory indicates that as of 1980, there were 396,211 members of the church, a figure that includes the Catholic populations of the District and the Maryland counties of Montgomery, Prince George's, St. Mary's, Calvert and Charles. But church officials admit that since there has been no census for 17 years, the figures are less than precise.

Yesterday's census form sought information on the marital status of the respondents, their standing with the church, if and where they attend church, where children attend school and the sacraments they have received, as well as racial or ethnic identification.

Most parishes in the archdiocese had distributed the forms at masses yesterday or earlier, with instructions to complete them and seal them into the envelope provided for pick-up by the census takers. To preserve confidentiality, completed forms are to be read and tabulated only by parish priests.

For the non-Catholics, the 300 volunteers from the huge Blessed Sacrament parish to which the Middendorfs belong, handed out a green mimeographed sheet giving the names and addresses of one synagogue and seven churches of other denominations in the area.

Middendorf, who has raised a family of 12 children on the income from his business of selling wine and candles -- mostly to churches --chatted amiably with everyone he met but kept moving on his rounds. "I want to get finished before 4 o'clock," he explained. "That's when the Redskins start, you know.

"People can get quite testy if you interrupt them while they're watching that."